Still haven't played Portal 2 and I still really want to. Everything I've learned about it though does point to the expected conclusion of "yes, its different from portal 1. Shame."
And it is a shame, really, because Portal 1 was such a runaway hit, and when something is that awesome we always wish there was more of it. The solution to the problem, of course, isn't to make a sequel: its to go back in time and make the original bigger and longer so that there is more space for the quintessence of awesome to permeate its entire being and thus further enrich our lives with the god juice that was the original Portal 1.
Thats not to say Portal 2 was always destined to be a bad game, just that it was, is, and always will be DIFFERENT from Portal 1. It is its own, separate game that, while rooted in conventions and mechanics of its predecessor, is still a separate animal entirely. Its like growing up with (in your opinion) the best damn dog on the face of the planet, and enjoying that dog so immensely that you jack its sperm/eggs and grow a new best dog on earth to replace the old one. The problem is that this new best dog on earth is going to grow up differently and will probably have a slightly deeper bark and maybe wag his tail a little more vertically than horizontally and instead of standing on his back legs with ease he is better at jumping and catching things. Its just a different animal from Portal 1, and because Portal 1 was such a fantastic animal its tough having to compare something that is at once derivative of its awesome but still separate from it.
I don't know how clear that all was. I still intend to play Portal 2 as soon as I can. I still plan to enjoy it, and I still expect to grow very, VERY tired of Wheatley before I finish.