The Good: Fantastic level design, completely original and wonderful narrative, tons of new game play mechanics, puzzles feel just right, looks good with nice effects, co-op campaign is just as good
The Bad: Looks a little dated, puzzles can be daunting for some, some weird checkpoint placements
Portal was a phenomenon and redefined how puzzle games are made. The narrative was original and also helped create one of the best video game songs ever created ("Still Alive" by John Coulton). Portal 2 helps build upon this in so many ways and really helps show how a sequel should be done. Instead of just being stuck in lab testing rooms through the whole game there are more environments to enjoy and the narrative takes leaps forward instead of just GLaDOS's voice droning on through speakers.
You play as the voiceless Chell once again, but you awaken after being in a coma for some time. A mysterious robot named Wheatley helps you escape your room and you finally get to see what Aperture Laboratories is like outside the testing rooms. You get to see glimpses of the outside world, vegetation has taken over the facility, and the puzzles now take form in so many different ways. Half way through the game you get to see what Aperture was like 50 some odd years ago and find out back stories on yourself, GLaDOS, and the founder of Aperture. The narrative is top notch with lots of cinematic moments, but everything involves puzzle solving and it's so mind bending that each one of the 50+ puzzles feels satisfying. It also goes to show how clever Valve is that I didn't need a walkthrough once to figure out any of the puzzles. There's so many "AHA!" moments that you just don't want to put the game down.
However, there aren't just portals involved, but other game play elements have been fused into the mix. There are three types of gels that you can use to solve puzzles such as propulsion gel (orange stuff that makes you go really fast), fusion gel (blue stuff that makes things bounce), and conversion gel (it allows portals to be made on non-portal surfaces). Due to these new elements you have to totally rethink how you work with portals and it's a great new mechanic and super fun to use. There are a few other things such as jump pads, new types of companion cubes, turrets, and other items to solve puzzles, but explaining is almost impossible because you should just play it.
The puzzles are just so uniquely fused with the narrative that you never feel like your just jumping from room to room anymore. This really feels like a puzzle/adventure this time around and some of the puzzles span two to three rooms even. There's constant chatter from characters while solving puzzles so it doesn't feel as stale this time around. All the new characters are great and you'll like them (or hate them) quickly thanks to Valve's one-of-a-kind storytelling abilities. The music is one thing I love about this game because when you start bouncing around a room or sliding there's unique music that instantly plays when you're doing these things. You get a strong sense of vertigo thanks to the game's new take on heights, but the music is just phenomenal. This soundtrack is purchase worthy and the new GLaDOS song at the end is just as good as "Still Alive" but in other ways.
The visual quality is great, but Valve really needs a new next generation engine. Portal 2 uses the Left 4 Dead 2 Source engine, so everything looks nice and there's some great visual effects (the gel mainly) as well as great lighting, but it's not exactly up to par. Thankfully there's a huge variety of things to look at this time around (the game is over 10GB!) and the addition of a co-op campaign with an additional story and two new characters is a huge plus so it's like two games in one. There's also some quality extras such as developer commentary and you can customize your robots in the co-op campaign which is a nice touch.
Portal 2 is truly something unique for this generation and probably the most innovative puzzle game ever made. With a strong narrative, lovable characters, and tons of new game play mechanics its like the first game didn't even exist. Don't be overwhelmed by working with portals because this game takes your hand for awhile and slowly lets you go when you feel confident enough you'll know. Valve knows how to make great games and other developers need to follow suit.