He's like an idiot savant. All he does is rap, rap, rap. On his albums, I think focusing on actual songwriting keeps him from focusing on what he's truly good at. When it comes to mixtapes, all he has to do is flow, so it's fantastic.
This is why hip-hop needs more producers and songwriters and less egos. All we have now is cats making beats and e-mailing them to a studio, or rappers without charisma writing verses for other cats. Imagine someone with the sheer rapping ability of Lil' Wayne or Bun B sitting down with someone with actual ideas about song structure other than a catchy chorus (Hi, 50!) or jiggy beat.
Shit, look at "Shooter". That was a complete song, there, Lil' Wayne just had to fill in the gaps with what he does best, and the end result was the most talked-about song from his last album.