Sackboy is so damn cute. Like, if I were five, I would totally want to go as Sackboy for Halloween. That character design is actually one of my favorites because he looks like a doll I might have had in real life that was made of sackcloth.
I actually came to LBP with the Vita version, and what people say is true about the physics. The jumping is floaty and imprecise. The level design for the game proper isn't too great, either. However, I love the music, the narration, and the real-world art-design. Don't get me wrong, I like the super-colorful design of Wooly World and Rainbow Curse too, but to me, there is room for both the more "realistic," I suppose I'd say (it's not the word that I really want, though) art design of LBP and the color-filled design of those Nintendo games.
However, I personally disagree with Sackboy not being a character that is particularly striking. Maybe what really got me liking the character is the fact that you can play with his facial expressions, and the animations for those are so wonderful that it feels like Sackboy has a real personality in a way that Yoshi or Kirby doesn't quite have, at least to me.