I finished Portal 2.
It kind of had the opposite problem to the first. In Portal 2 the dialogue in the beginning is good but then plot things happen and Glados becomes a completely different character, they destroy what was good about her.
Gameplay wise it's more of the same but for longer.
As for the story and setting, the first one barely had one which I think was preferable, I feel like they tried to do too much here.
I feel like much of the reason Portal has such popular appeal is due to it being old(relativity) and made by a well known company, I think that if it came out today people would not find much of interest about it as there are so many indie games that do things like what they do only better.