Seemed like a pretty definitive ending to me, though, Chell finally making her way out of the labs, cutting all ties (besides companion cube, <3) and making her way out into the real world again.
And I'd certainly be disappointed if they had a sequel involving Chell in the works.
The only negative thing that I could mention about the story mode (besides the head-scratchingly obtuse level designs with absolutely gigantic rooms) was that the climb back up from the bottom might have taken a tad too long. Don't get me wrong, though, I liked the way Aperture's history is explained, and Cave Johnson makes for brilliant commentary, but there are a couple of levels that offer absolutely nothing besides another 10 minutes of jumping around and firing portals and mucking about with gel.
And I don't mind that, except that the first Portal game was short and sweet enough to get away with that, while the second game is so big that it becomes more obvious when there are levels that exist solely to add more hours to the game, because they stand out so much from the levels that teach you something, or the levels that further the plot, etc.
The sizes of the levels do probably contribute to the feeling I have about that, of course: the first Portal's rooms were tiny compared to the majority of rooms in the second game; only perhaps one or two of the last levels were even comparable in size. HUNDRED-FOOT DROPS EVERYWHERE etc. There's a great thing about the tangible bigness of the Portal 2 levels, but when it makes you realise exactly how long it takes to traverse them, well.