good Userinterface,
polishing,
a well balanced rewardstructure
and style-guided art
(where the way the style of all elements fits together is more important that the quality of the single assets)

are the major points in my opinion.

Stories are easily told, and most of the time ignored by the player.
The player will often listen to the story more, when he has fun
with the game itself.
Most games use known gamemechanics anyhow, with a
twist here and there. So dont bother if it is not totally new.

Often innovative games just FEEL new, by bringing a common game-mechanic into a different perspective.