Achievements: Some developers do a better job with them than others. i.e. The flags in Assasin's Creed are lame. But I have found myself rescuing all the citizens and climbing all the towers in a city just for the achievements, I probably wouldn't have done that otherwise.
Some games, such as Mass Effect, actually do attach unlockables to achievements. For example, if you get the $1,000,000 credit achievement then you can buy special weapons you normally can't. Or if you get "I killed X badguys" achievment your charcter earns bonus XP when fighting them in the future, etc.
I agree, I'm not into "I am awesome because I have a gamer score of X" but it is fun to see if a friend has made it as far as you in a game, or has finally nailed all those songs on hard in GHIII etc.
File Size limit: The file size limit is for arcade games mostly because (I think) of MS's desire to support 360's with no hard drives. They want arcade games to fit on the memory sticks. It used to be very tiny (50MB) and was recently upped to 150mb. I expect we'll see this number grow over time.
Gamers have 3 choices this generation: The Exercise Machine, the Movie Machine, or the Game Machine. What kind of gamer are you?