So I installed the beta this morning and played for about 15 minutes before heading off to work.  I didn't get to see too much, but so far, I'm impressed.
Purbeast is right, the physics do feel a little floaty, but I don't mind it so much.  I'll get used to it, and soon I won't even recognize it.
I love the voice-over.  There were actually more than a few moments where I laughed out loud, because the wife was in the bedroom fixing her hair, and was asking me what I was laughing at.  The dialog is pretty clever, and the voice is perfect for it.  I also loved the opening romp through a level that showed everyone's names and pictures.  It was very well done, the music was great, the small amount of interactivity was perfect...just a splendid job.
I was also pretty surprised by the extra personality you can give to your character by changing their facial expressions and using the sixaxis to change where they look.  I don't know if these things are used in actual gameplay mechanics, but even if they aren't, they are welcome additions just to get a laugh by making your in-game character express your feelings.  Like in the tutorial level where you call into big pit, so you put on your scared or angry face, then look up in desperation.
I also like the idea of collecting all of your "accessories" instead of just having a huge assortment from the very beginning.  At first I was wondering where all the options were, but it didn't take long to find out why.
As far as the graphics are concerned, I think they are pretty much perfect from what I've seen so far.  I'm running at 1080p, and while I've read some things about aliasing, I've not seen much if any of it.  The beta runs smooth as silk.
In my short 15 minutes this morning, I think I may fall in love with this game.  It seems to want to bring out my inner child 

.  Tonight I'll actually go through some tutorials and get into building a level and see how everything works.
EDIT:  OP, you might as well make this the official LBP thread since it releases in 2 weeks.  I'm sure we'll all need a place to post our crappy levels we make 

.