Unimportant things that you miss.

[BreakABone]

I agree with the trophies/achievements comment.

They are effective when done well, but sometimes become more of a chore than anything. Especially when developers put in great ones like the Blast Shards/Orbs from inFamous/Crackdown where you are combing a giant city for one tiny object.

Or online enabled ones, especially for games that won't have stay power so if you buy the game later on, almost impossible to get. Or the ones like winning 20 ranked matches in a row because someone will disconnect on you or something.

Or ones like playing with someone who has played with a specific player and/or developer.