I loved Portal 2! Already beat it in single player and co-op so I wouldn't have minded a bit more fun, but I enjoyed it.
I was concerned about Portal 2 living up to the high expectations set by its predecessor, but Valve pulled it off.
As ya mention them specifically, I'll admit the old HL2 doors stood out a bit of a sore thumb, I'm not even sure the flat texture changed on those other than the color. Forget comparing anything to the consoles, those doors predated the 360 and PS3.
Some of the other flat textures looked off if you stop to look at them as well, but overall they blended in well enough. Hopefully Valve will decide to make a game that doesn't involve those flat texture doors, they are ubiquitous given they exist in every Valve game since the original Half Life, other than perhaps TF2.
Don't click this link unless you've finished single player mode already. I set the vid to start later so it doesn't give away how to find this for those that missed it on their first playthrough.
http://www.youtube.com/watch?feature=player_detailpage&v=C0lKE3GAosQ#t=34s