First of all, I want to say that I pretty much liked it.
And because I liked it, I'm going to nit-pick it 
So... my off-hand thoughts as I played through it:
- Opening cutscene seems jagged in one part, in that, if I remember correctly, when Link walks left off one screen, the next screen shows him walking upward. An additional screen showing him turning upward would flow smoother.
- If possible, I would suggest having Link start with the boomerang as his inventory item rather than the bombs. I tend to press keys after cutscenes to be sure I'm in control, and I accidentally planted a bomb while doing that.
- The music can get overbearing at times. The Lost Woods track seems rather hyperactive compared to the setting it accompanies. The high-pitched ostinato that overlies the entire track reminds me of the frantic music that plays in the old detective-mystery game Deja Vu when your character discovers the dead body in the office, and the way it keeps going on and on can get tiring after too long. Something slower and more mysterious would suit the scene better.
Along the same lines, when you take the stairs down to the basement in the first dungeon, the Level-9 music is at least twice as loud as the first-floor music that it switched from, if not three times. You should lower the volume significantly, because I could barely even hear sound effects down there.
Also, the overworld music briefly stops after a few minutes before restarting. Not sure if that's a loop configuration issue, or just the file itself. Either way, it's worth looking into, as it makes for a somewhat awkward jolt.
And now, an illustrated commentary!
- Waterfall consistency:
So I noticed I can swim really far into these waterfalls. It looks a little strange with the dive function, as if Link were diving sideways into the fall. I was going to say he ought not to go so far into that first tile, but then I saw this:
To me, this looks like a lot better swimming limit than the two waterfalls above. I think it would look best if all waterfalls had something like this as their swim limit.
- The Lost Woods:
Music aside, I think the place has a very nice atmosphere and ambiance. Even though it doesn't have the cloud overlays like LttP, the dark and dense colors make it look like dark rain clouds are lingering overhead. An appropriate look, given what's going to happen next.
This probably isn't anything you can do about, but I notice some of the tiles tend to jump between smooth and straight edges. For example, in the first shot above, below and to the left of the waterfall, you have those nice smooth wavy-edged tiles between the blue and green floor areas - but then you put those animated flower patches in the corners, and now it's strictly straight-edged. There's a number of variations like that throughout the forest, and some later on in the rest of the quest, that make minor interruptions in the natural flow of these places. If you have the tiles available, I think tweaking these things could go a long way. But, if you have to draw them manually, I don't think anybody's going to blame you for leaving it as-is.
Well, except maybe Jared 
As for gameplay...
Being new to this manner of quest, I went jumping down all these holes trying to get to the other side before realizing how these layers worked.
I'm an idiot
However, speaking of jumping down holes, I noticed something odd regarding the holes below the button. If I slip between the two to the left of the button while holding the up arrow, the screen begins to scroll upward, before snapping back down when Link respawns. It may just be an engine bug, but I thought I'd mention it anyway.
And then, on that next screen up, I walked into an armos and died as it began to come to life. There's a Quest Rule, the second under NES Fixes, that says "Can Safely Trigger Armos/Grave From The South." That was a big annoyance for a lot of people, so I'd recommend checking that rule, especially since these ones can only be triggered from the south.
Also, maybe the button should play a sound when pressed?
- The flood scene:
Really nice animation sequences. I like how all the different scenes gradually fill up with water.
Heh, actually, when I was really young and doing the coloring book thing, I'd always get yelled at, because, instead of properly coloring the stuff, I'd just grab the blue crayon and draw the scene gradually flooding until everything was submerged. This was a nice throwback to that
However, a note about the dialogue:
First we see it during the daytime. OK, cool.
Then it shows again during the rain scene. Kinda redundant.
Then it shows a third time during the blackout.
If I had to guess, maybe you meant for a single line of text to show per screen, but it showed all three for each screen on accident? I think that might be the best display option, that way the screen display length could be consistent.
(Also, it's "engulfed.")
- The first island:
I wasn't sure if these doors were supposed to be openable or not. I thought not, but then I didn't seem to have every item I needed by the end, so it made me wonder if I was just too stupid to figure out how to open doors.
Transitioning between these screens, though the two corners I'm standing just below look fine at the edges of their respective screens, they look awkward during the screen scroll, as if there were some tiles missing between them.
You seem to have some layering issues here, as the tree trunk and roots are being drawn on top of Link like overhead tiles.
Same here.
Not really a "problem," but Klaudia kinda talks like a neanderthal
Here, I tried to open the chest I'm standing below. All six of them opened, and none of them had anything inside. Intentional? Or was I supposed to get something?
Also, that place in the middle of the 4 pillars looks like I should be able to go into it, but I couldn't. Intentional again? The place looks like a 2-story building from the outside, so I figured it would have an upper level.
- First cave:
That should probably be a staircase up, not down.
Just carrying on from the Lost Woods tiling thing, it looks like these rock walls were made to transition smoothly into the floor just before the actual floor tile was placed - but the color along the edges and corners doesn't match the rest of the floor. Maybe it's a different CSet? I don't know, it just stuck out to me.
After I walked up to this screen, I couldn't walk back down.
Only one half of the pass-through is overhead. I'd suggesting making both or neither of the tiles overhead.
That said, I really do like these kinds of transitions between overworld areas. Really LttP-ish.
- Waterworld:
I really like the idea. At the moment, the map seems rather barren, with so many blank screens of extensive swimming. It does add an atmosphere, and I like all the different islands scattered about, but such a wide open scene like this seems more geared toward LttP's larger scrolling areas, rather than ZC's screen-per-screen setup, particularly...
...when it comes to things like this. See, I'm transitioning between this screen and the island one to the right...
...and now that I'm fully on this screen, look to the left and see what I mean.
The rock clusters are against the edge of the screen, but they don't continue into the next screen. As a result, I hit an invisible edge at the side of the screen to the left, and I have to keep swimming along the edge until there's a space on the current screen that allows me to pass over.
Another example comes here, by the big raft path. When I swam down into this screen, I ended up inside a rock cluster, and when I tried to swim out of it, I ended up inside the raft path...
...and then if I go up, I'm on the other side of the guy's house, without having picked up the raft from him.
Here's a better example of what I'm talking about. This is me at the edge of one screen, transitioning into the one beneath it...
...and now I've passed through the rocks, and am back outside of the raft path again.
This happens all over the place. Sometimes it happens on the land areas too, with bushes and trees, but it's especially prevalent while swimming around. My suggestion is to extend the rock clusters into the next screens of the sides they're touching. Not only does it fix the transitioning issues, but it gives some distinct detail to otherwise blank and anonymous water-filled screens.
One last thing - it's weird hitting invisible edges. I may be alone, but it's my personal preference to always give a solid reason as to why the player can't pass any particular barrier. In this particular case, I'm not sure you could do that without making the entire overworld look like one giant mountain-top lake - but perhaps you could expand on those rock clusters you've placed around the middle and work them into an edge-wise barrier as well.
That said, I understand this is just a demo, and perhaps a lot of these things I'm commenting on are in progress already. Again, these are just my off-hand thoughts from a first play-through, don't take it overly-critical.
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- First dungeon:
Link enters facing down rather than up.
You should make overhead versions of the solid tiles at the top of this walk-through to prevent clipping oddities like this.
Kind of tedious, I know from doing it myself, but also quite worth it in the end.
Unattainable key in unseeable room keeps me from making progress after the bridge.
This, along with an unlit torch lamp on the second floor, makes me think there should have been a candle somewhere along the line, but I was all over the overworld and the caves and houses, and eventually the dungeon, three or four times and never came across one.
Also, note an overhead tile at the top of screen cutting off Link's sprite.
I absolutely loathe the missing 8 pixels at the bottom of these screens. Most of the time I just deal with it, but in situations where it cuts off important parts of the design, I would actually design around it. Here your torches are getting cut off, so I think bringing the wall up a tile would be a good idea when issues like this arise.
A lot of these spinning tiles disappear when they begin to move. No idea why.
Could be the shadow effects causing a problem? Those remain even when the tiles disappear.
Eh... I guess it's just one of those kinds of quests.
Would be nice if things like this could stay solved though, with the number of times I backtracked through here.
Also, when the text comes up, it should be spelled "basement."
Well, I can't buy anything at all, because the interaction ends after this text line.
Maybe this is where I was supposed to get a candle?
Anyway, that's as far as I could get.
All in all, it was a fun demo with some nice design scattered about. The apparent lack of a candle, and my extensive search for it, put a damper on the adventure. But, it wouldn't be the first time I've missed something blatantly obvious. It just makes me wonder, if it was supposed to be in one of those empty treasure chests, or if the merchant at the top of the dungeon was supposed to have it in his shop. For what it's worth, I didn't successfully bomb anything open, despite the amount of bombs I started with and gathered throughout the demo. I may just not know what to look for with this kind of tileset, while things like that may be totally obvious for people familiar with it.
So... like the idea, like the story, like most of the layouts, like the difficulty level... just needs a bit of fine-tuning and I think you've got a good start. Looking forward to future updates.
