I am, sadly, reminded of The Matrix and it's sequels. The Matrix was almost a sleeper hit that came from no where and took the theaters by storm. Then when they announced a sequel, no matter how good it was or could have been, it now had to compete with The Matrix. I fear, a bit, that this is what one will find with Portal 2.
Not because Portal 2 will be bad, lack luster, or even sub par compared to Portal. It may, in fact, be twice as good, but people will expect the same feelings that Portal pulled from them.
Instead, I predict people will get the 'standard Valve procedure': "More of the Same, But Different". It will be Portal, it will give more physics flinging, ball busting, Sentry centered game play, but they will somehow up the anti.
We've learned from past franchise games that sequels are like that (Mario, for example). It's not bad, but people are going to ruin it for themselves. Me? I'm going to buy it regardless, because like the player made maps you can download, it's that kind of stupid fun I love.