Look at the user reviews on metacritic. There is a pretty large disparity between professional reviewers and normal users. As far as campaign length yes, Portal 2 is a lot longer than Portal 1. But the first portal was also part of the orange box and the individual game was sold for a much lower price. I think Portal 2's total gameplay comes out to about 15 hours, and from what I understand that is pretty generous estimate. The problem for me is that that isn't a whole lot for a $50 game, and it doesn't have much replayability after you beat both campaigns.
Well have you read the user reviews on Metacritic? Almost all of them give the game 0 because of the dlc and because the campaign is 4 hours long (which is a lie). Is it fair to give the game 0 / 10 because of those reasons? definitely not.
15 hours is rather long for a new game these days actually. :shrug:
also you should read this post on RPG -> http://www.rockpapershotgun.com/2011/04/19/lets-address-some-portal-2-nonsense/
I also think that Portal 2 has a great replay value. I spent all day at my friends house playing it (I don't have it, yet) and after finishing the singleplayer, which took about 6 hours, I started the game again and tried to do it faster. The game is perfect for speed runs and challenges.