I just bought it even thought I already own Bastion, so I've got a spare key for that. But I must agree, this is the best Humble Bundle so far and I've only missed the first one.
Bastion is a must play, if you have not played this then you have missed something huge in your gaming life. The way that the story is told is not only innovative but also probably the best way to tell a story in a game ever. And then you can add the art style and the soundtrack on top of that. Heck, the game is worth getting it for the soundtrack only (I chose to buy the sound track after I played the game). The gameplay is also very good and there are loads of cool ideas and innovations in it like the shrines which allow you to customise the difficulty level in the way that you want and you can change it in between levels. It is on so many best game of 2011 lists and usually it takes a spot in the top 3. It really is a must play game.
Then there is Amnesia: The Dark Descent of course. This is one of the most played games of last year as it is a really scary horror game. Just look up Amnesia on YouTube and you'll find loads of let's plays where people scream until dawn. Personally I have played only the first 5 minutes at a friend's place and I was not that impressed (it was before I heard about the rest of the game though, my friend was just like: "hey, play this") but most people are already on edge within the first minute. It is that scary!
Limbo is one of the most acclaimed indie titles and I got the bundle mainly because I haven't played it yet. But the visual style is so cool and really attracts me. On top of that it is also very unforgiving and it might kill you out of nowhere. Instead of fighting something in this game you have to get around the obstacle in some way. It really is one of the best platform puzzle games that has been released in recent years. Just beware of the spiders!
Psychonauts by Double Fine IIRC is a very old game which is getting a sequel by Notch. I have not played it myself and I am not to familiar with it but everyone that I know that has played it was very enthusiastic about it. I think it is even Yahtzee's best game of all time IIRC, which is saying quite a bit as he barely likes anything.
Sword & Sworcery is also something I don't know that well, but the graphics style is really good. It seems a bit weird in mechanics though as it is ported from the iPad to PC. It also has twitter integration so your tweeps can help you solve stuff etc.
All in all the bundle is really worth it and you should get it, even if it means that you won't be able to pay of your debts in the following 10 years