Tilesets: It's neat if a game has custom tilesets and graphics, but they're more of a bonus. I can still enjoy a game that just has the default as long as I like the other qualities of the game.
Sprites: These are important. If a game just uses pre-existing trainer sprites for important characters, that's a major turn-off. I want to know what the rivals, gym leaders, etc. are supposed to look like.