The main reason that people powerlevel is that the game forces us to be become level whores. I have been playing the game for almost a year, as a casual gamer, and the game really hasn't changed that much. Everything that they have added is directly related to levels, the only reason to play the game is to level up. That problem, yes its a problem, can be hung squarely in the devs closet. If there was something to do that was special and fun at certain levels, and only those levels it would slow down PLing, but the only thing to do when you level up is get new enhancement slots or a new power. There is no reason for anyone to sit around and grind away if there is an alternative, you don't really miss much, just cookie cutter missions and perhaps a badge but exemplaring(sp?) took care of the missed badge thing. If the devs took the time to make the game fun at all levels then I believe that most people would play the game instead of PL. I also believe that there are a certain segment of the gamers that will PL however they can and there is nothing they, the devs, can do about it.
Before anyone tells me to play a different game, I would like to point out that I love this game. It has, in my opinion, a tremendous amount of potential that is still untapped.