This is silly. A review for Portal 2? Why? The game has a known quantity of fantastic – it's a lot of fantastic. Did you expect less? Unrealistically expect much more? Portal 2 is fantastic, and, I suppose, if you want a reaffirmation of that statement of fact, then sure, read on.
Valve did a lot with very little with Portal. No more than ten people were involved in Portal's development; the result was a concentrated experience that managed to surprise everybody because it was assumed that beyond the collection of puzzles, there would literally be nothing else to it. What was thought to be a novel experience became an unforgettable, inventive, eerie, and hilarious, usually all at the same time.
With Portal 2, the cat's out of the bag, the element of surprise is gone, and so Valve simply does a lot with a lot. With a full-fledged team, Valve blows up the scale, has more than one character with a speaking role, and presents a game that's bigger and mostly better. Plot and story are crafted a lot more traditionally, with multiple characters talking and interacting with one another and a much more apparent three-act story arc. It's roughly the same scenario as Portal; you're still Chell, and you still need to escape the facility, but you're given a lot more help as characters tag along with you for most of it.
"…it's a lot more adventurous and lighter…"
Tonally, though, it's a lot more adventurous and lighter, with less of the black humor undertones that gave Portal a remarkably memorable atmosphere. The game's still brilliantly written, with extremely naturalistic and hilarious dialogue that fits with Portal 2's new tone. It's a game, just like Portal, that's going to be remembered more for its quotable lines and newfound memes than the gameplay itself.
Yet the game plays brilliantly. You'll notice all the things that Valve tends to do with all their games – slowly acclimating and training you to new types of gameplay, building on foundations before putting everything you learned to the test. It helps Portal 2's puzzles be as satisfying as they are, where jumping across chasms, painting the world with gels, and traveling through gravity funnels is a typical day of science for Chell. It's the kind of madness that – once the solution plays itself out, after you've properly placed your portals – makes you smile widely.
The puzzles are even better in the co-op game. With four portals in use, the complexity and ingenuity of the puzzles increase, opening up possibilities that couldn't have done in the single-player game. You'll need an able partner who's willing to deal with… people, but once you find that special someone, it's really an incredible experience.
The only real problems come from the technology Valve is using to power the game. The Source engine's output is really something to behold in Portal 2, pushing the lighting and shadows into levels never before seen from this engine. The game looks fantastic and full of visual variety, from sterile test chambers to industrial complexes. Visually Portal 2 stands with the best.
The problem, then, is the excess of loading screens. They're alarmingly frequent, disturbing pacing especially in the first third of the game, where a load screen presents itself after nearly ever single puzzle. Couple this with the loading screens themselves, which abandon Valve's minimalistic LOADING sign in favor of images showing up during loads, and what you get is Valve's least immersive single-player game.
That's really the one true mark against Portal 2, because the rest of it is carefully structured and designed with a level of expertise that you don't see that often. Valve knows when and where to implement dialogue, so that you get the full effect of a story beat; they know when to relent with the puzzles and ease off and let you take a breather. Okay, maybe the middle of the game is a bit weaker than its first and last third, but if you're looking to really rip on Portal 2, you're just going to sound like a lunatic.
To reiterate, Portal 2 is fantastic. Its single-player provides ample amounts of genuinely funny and compelling narrative and some great puzzles, while the co-op takes the game's mechanics and pushes them to their absolute, playable limit. You're only getting around 10-12 hours after delving into both halves of the game, but it's a concentrated thrill, where its weakest moments are still standards that most developers can barely reach in the first place.