make sure its challenging, or people get bored to quickly. make sure its not ridiculously hard (unless its the unfair platformer) and MAKE SURE IT HAS GOOD GRAPHICS. and people hate lag. oh, and make more than one game mode. like a story mode and, hmmm...
oh yeah, if its a shooting game it can have a practice range. like, stand somewhere safe and blow everything up.
errrr...... im outta ideas