This one's definitely a big one, and one of the easiest to fall into because you don't even realize you're doing it. For example, we all know I'm a massive IoR fanboy. Well now, making my own quest, I keep finding myself subconsciously trying to constrain myself to IoR's guidelines, which does nothing but hold me back at best and make my work seem like a cheep imitation of Evan's at worst. There's that fine line between taking inspiration from somebody and copying their entire design structure, and it's really hard to stay on that line at times.People trying way too hard to mimic critically acclaimed quests/questmakers - I know some may feel pressured to create quests that live up to the standards of critically acclaimed quests in our DB so that they can have that kind of glory too. While there is nothing wrong with drawing inspiration from other quests, no one should be restrained by what critically acclaimed quests offer. In fact, if everyone were to mimic each other, then ZC would most likely end up like all the other milked franchises in video games.
Heavy Scripting - While it's true that some scripts actually enhance a quest to allow a cleaner gameplay experience. Some may go way too far with scripts that it does the opposite of what it's intended to do. Scripts != quality. Not one bit.
I wouldn't say this one's necessarily bad. Scripting is a tool, and like any other tool, it can be used for good or evil. I've seen quests that rescripted everything and were quite enjoyable for it. Conversely, I've seen quests that threw in lots of scripts for the sake of having lots of scripts, and they came out worse for it. It really just comes down to implementing scripts well. If they're used well, it all comes out fine. If they're thrown in just so the author can say the quest has scripts, it usually shows and drags down the quest.
Now a lot of stuff that I'd say has already been mentioned here, but let me throw in a couple of others bad design trends.
Completely Pointless Rooms. Don't get me wrong here; not every screen can be hugely important. Sometimes you just need a basic hallway connecting point A with point B. That's perfectly alright. But I've seen quests with passages dead ending in a room that serves absolutely no purpose. What's the point?
Boring Enemy Gauntlets. This is another one that needs some clarification. Having to kill enemies is good. Having sections of dungeons when you need to fight through a gauntlet of enemies is also alright. But I've seen entire dungeons that amount to nothing but endless kill enemies->open shutter rooms, and that starts to get boring after a while. Especially if you're just using default enemies. If you're going to make a use enemy gauntlet, at the very least use some custom enemies.
Unfair Difficulty. There's a difference between true difficulty and fake difficulty. Having a few challenging enemies that you have to tread carefully around can be very good. Throwing in lots of death knights and expecting the player to just tank through it is bad. Really, any time tanking is the main strategy is pretty bad.
In short,

