Friendly AI has to adapt to the player's movements and orders. Enemy AI has to adapt to the player's behaviors and learn accordingly. Those are the fundamental rules of good AI. Game programmers can take a lesson from the military AI in the original Half Life. They flank, flush out, throw grenades when the player is hiding, and adapt.

Gameplay has to be precise, but not touchy. Crate jumping puzzles are fun, but only when they are sparse. Optional exploration of a level is vital, but not fun when every level looks the same (Assassin's Creed). Innovations to new ideas are good, so long as they integrate themselves into the game well. But most of all, gameplay has to be FUN.