The only things I've seen mentioned that I would recommend are Cowboy Bebop, Tengen Toppa Gurren Lagann, FLCL, Yu Yu Hakusho, or One Piece (the latter two are better if you like long, multi-season shows). I would not recommend Naruto or Bleach unless you like the same things over and over again for a billion seasons, but if that doesn't bother you and you're interested in the whole Ninja / OP main characters thing, go for it.
If you enjoyed Avatar: The Last Airbender, you should check out Avatar: Legend of Korra! It's only one season at this point, but the action is AMAZING. The romance is eh, but you might like it more than I did. I'm not big into romance.
If you want a taste of something a little different, I highly, highly recommend Mushi-shi. It's on Netflix (but I'm a purist and think subtitled anime is best anime so... hit youtube up, haha), and it's about a traveling 'exorcist' of sorts who solves problems related to mushi, or other-dimensional spirits. It's beautiful, and it's peaceful at times when there isn't awesome action. It isn't big on humor, though, which is why I recommend...
Samurai Champloo! Another recommendation which I haven't seen yet, and am shocked at the absence of mention. It's funny, the characters are very dynamic, there's action, and the art style is awesome.
That's all I got.