To add to the list of Lee-traits that this thread has collected:

He's funny, he's so sweet to Sakura, he's brave ... he's the most adorable character in the series (I SAY SO), he manages to be/act cheerful in spite of his bad situation, he has a great smile, he's kinda clueless (? i dunno, like before his Gaara-match when he was discussing with Gai) ... and everything else already mentioned like hard work/dedication/strength ^ˆ^