One more thing: there are many tile sets out there that consist for example of some nice platformer tiles, and then one beautiful hand drawn prop like a tree. If you use that tile set to create a level, you'll see that one tree over and over again. I much rather have some tree parts that connect in different ways, that can be used to create an infinite variation of trees, even if they're slightly less beautiful. It seems that some artists (especially for free tile sets) are focusing on creating a single nice painting, instead of creating a useful tile set...