There is now incentive to build towns smarter and in a more interactive way.
- NPCs don't like to be crowded.
- They like to be near specific NPCs and away from other specific NPCs.
- Decorations don't really matter but not decorating at all might make a negative difference.
If you build houses for three NPCs within a specific biome (I.e. a desert biome, a forest biome, a snow biome, etc.) and make them happy, they'll have the ability to give you a Pylon as well as offer cheaper prices on things. You can place one Pylon in that Biome, and only one in each kind of biome, (So if you have two Desert biomes only one can have a functioning Pylon.) and then you can warp between all functional Pylons as if they were instant teleporters.
That means you now have a reason to set up functioning tiny towns all over the place in each biome and scatter them around the world. Easy early teleporting!
My one hope is that it doesn't mean unhappy NPCs charge more rather than just not charging less when they're happier. I think they will charge more when they're all cramped together, yeah. At least according to Leinfors, one of the Terraria staff.