Mine is his personality. As the authors of Death Note described him, "A young man who plays video games and doesn't care much about the world."
Odd as it may sound, I like him because in a way he seems to represent people like me who would rather be lost in their own little worlds than deal with the realities of life.
Of course, this doesn't in anyway reduce his badassery in the series. One of the best things about him is that he is one of anime's most popular characters of all time, and he only appeared very rarely in the story!
What about you guys?