I'm going to make my own list here.

Here are the things I feel MUST be in any tileset, no matter what.

-Multiple types of ground tiles
-Sloped tiles, preferably at two or more angles
-Sprite layer background tiles
-Other background tiles, enough to fill layers 6, 7 and 8, and optionally layer 5, although this isn't as important since many people prefer to use layer 5 as a background to layer 4.
-Plenty of eyecandy, including animations.
-Vines and hooks
-Destroyable blocks
-Spikes or other hurt event type things
-Poles
-Signs
-blabhalvhavjkhsdflgkajhgdashgd too lazy to finish list