I don't know if anyone at Valve ever really intended for there to be a sequel to Portal before it launched. The original was more like a very ambitious Half-Life 2 mod than a stand-alone game proper, thrown into the Orange Box to add a little extra value to the package. Then it blew up with "CAKE LOLOLOLOLOLO" all over the internets and a sequel to collect the moneyhats all the fans were lining up with was pretty much requisite. Under the circumstances, I think they did as well as was possible. The single-player campaign does feel a bit padded and uneven when stretched out from 3 to 8 hours, but not off-puttingly so, and there's some decent twists in the gameplay mechanics thrown in to freshen the experience up, as well as a pretty good expansion of the first game's rather threadbare story.
If you played Portal years ago but haven't really looked in or thought about it since then, you might be understandably confused about the setup of the story. Back in late 2010 somewhere, Valve snuck a patch into the original game via Steam that adds a bit to the ending where heroine Chell gets dragged back into the Aperture Science facility by a robot, whereas in the original (more ambiguous) ending it appeared she had escaped. So she's put back in stasis and years go by, finally she's woken up by Ricky Gervais Bot who's apparently sick of his job and wants to bust out of the joint, helping Chell along to find a new portal gun and navigate a bunch of test chambers to try to get to the surface elevator in GLaDOS's room. Of course, bumbling GervaisBot manages to re-activate GLaDOS, and we're off on a fun new round of Portal testing for science.
The game brings back all the puzzle elements of the first game, but Valve found a new Digipen project to swoop in on and gank ideas from (Tag: The Power of Paint), so about halfway through the game you get the addition of random pipes full of different colored gels everywhere that you have to manipulate. Blue gel makes you bounce higher, red gel makes you run faster (usually for launching off a conveniently placed ramp), and white gel turns a previously un-portalable surface portalable.
8 or 9 more hours of GLaDOS testing you would get old, of course, so the new twist here is that you'll spend roughly 1/3 of the game exploring the forgotten caverns beneath Aperture Science, where the gels are found amidst the ruins of the original laboratories from the 1950s and 60s. This also provides the game's central story, in developing the backstory of Cave Johnson, nutty "visionary" behind Aperture who was only mentioned in passing in the previous game. These "underground" segments manage to be both interesting and tedious at the same time. They sort of resemble the "backstage" parts of the first game, except they're MASSIVE, having an almost Metroid-like vibe and exploration quality at times. Unfortunately, the means of exploring them is to peck around these giant rooms zooming up and off into the distance looking for obscured bits of portal-able wall, which is the tedious bit.
The game seems to sometimes get a knock for being "dumbed down", but I actually thought it was a bit more challenging overall than the original game. The easy puzzles are REALLY easy, sure, but there were three in the later stages that I had to duck out and look up a walkthrough for, which is three more than I needed to in the first game.
When you're done with the single-player campaign, you've still got a substantial co-op campaign in front of you. That is, provided your Steam connection works better than mine, where it constantly waits forever when trying to start a game (does this with Hat Fortress 2 for some reason also). Also, provided you can find someone still new to the game to play with. Really, at this point (a year after release), if you don't have a friend who starts the game at the same time as you, co-op is really almost inaccessible. The strangers that you can hook up with playing online are nearly all vets of the game who have beaten the co-op multiple times already, know how to solve every puzzle, just want another vet to go along with them so they can get the Dr. Peenis Cheevo or whatever the hell it is, don't want to be slowed down by a newbie looking to reason their way through puzzles, and will bark orders at you if you stop to think about something for more than a second or two. There is also one free DLC addition at this time, but it just adds another chapter onto the co-op experience, doesn't help you much if you're walled out from that for one reason or another.
If I'd paid $40-60 retail for this a year ago when it first came out, I'd probably have left feeling a little Sharfted, despite enjoying the single-player campaign. Though maybe I'd have actually been able to play co-op then. At $7.50 over the holiday sale, however, I was quite happy with it. Portal 2 doesn't do everything absolutely right, but it does nearly everything very well and a couple of things perfectly.
* >Gameplay Video