Superior to the first in every way.
Portal could have been great. The concept was ingenious: a gun that shoots portals is a very cool thing indeed, and allows for some amazing level design. Unfortunately Valve screwed up by making the puzzles way too simple – the whole game felt like a long tutorial for a set of harder, better puzzles that didn't actually exist. Enter Portal 2. It's got a plethora of new puzzle elements: tractor beams, cubes that redirect lasers, a gel that makes you bounce, a gel that lets you move at lightning speed – it goes on. The trailers make it look fast and complicated. It seems to have precisely what was missing from the first Portal: challenge.
Problem is, it doesn't. The new puzzle elements – the tractor beams and the gels? They're cool, but they're never combined in especially intricate ways. Even the most advanced of the puzzles are extremely simple (in practically every puzzle chamber, you can see the solution simply by taking a single look around the room) and like the original game the entire experience feels like one long tutorial for a harder, better game that is nowhere to be found.
Aesthetically, on the other hand, Portal 2 is a large step forward. The animations are very impressive indeed (to give some examples: GLaDOS and the new robot Wheatley are both extremely expressive despite having neither extremities nor facial features save for a single eye, and the way gel jiggles and deforms when caught in a tractor beam is incredibly cool). The environments, which were fairly boring in the first game, are now far more varied and interesting. The facility starts out ruined and overgrown and is restored as you move through it, and you get to visit a number of new areas including the oldest, most primitive parts of the Aperture Science complex.
The one aspect of the game that is truly exceptional, however, is the voice acting. It's absolutely amazing, and wouldn't seem out of place in a Pixar film. Regrettably, the characters' banter is ruined by strained writing – the game constantly tries to be funny, but for every good joke (and there are some great ones) you have to listen to dozens of lame, at times annoying ones.
Like its predecessor, Portal 2 could have been great. It's superior to the first in every way: it's got enhanced animation, more and better-looking environments, stellar acting and new, cool puzzle elements. Unfortunately, the parts that have been substantially upgraded – graphics and voice acting – are also the least important ones; the most critical component, the puzzle design, was only marginally improved. There is incredible potential in the Portal series (now more than ever, with all the newly introduced mechanics) and I would love to see it fulfilled, but as long as the developers remain incapable of or unwilling to create truly diabolical puzzles, there is simply no chance of that happening.