I stumbled into this interesting little essay/page back when Death Note had yet to gain that much popularity.
For the most part, I tend not to attach symptoms/titles to fictional characters especially when the author hasn't even explicitly proved or disproved the "fan given title." However, once the anime for Death Note came out, I noticed that they really played up L's "characteristics" like his sweet tooth, and how he picks up objects. For those who've seen the 3 hour special, there's also a scene where L washes his hand with a cloth immediately after shaking a certain someone's hand.
I'm personally convinced that L, at the least, has some kind of mental condition that could be autism. Thoughts? Also, would you say that Near and Mello have a similar condition?