He was never one of my favorite characters. Overdone angst and tragedy get on my nerves. He wasn't even an especially good looking male for the Bishi dominated anime industry. I actually thought that (*grimaces, dislike for the Starlights becoming more evident*) that Seiya was better looking. His towering nobility is somehow nauseating, while it works for Belgarion the Godslayer (*nods towards David Eddings*). Furthermore, he's a pretty boy who doesn't really have much by way of powers. You try defeating a youma with aurlic healing, mind control, psychometry, and a few thorny roses!
Yes, it's sweet that he would break up with Usagi to keep her safe, but this is the same season as he gets brainwashed and starts kissing a VERY warped Rini. He gets brainwashed on an average of one a season (hey, three out of five isn't bad!), and he started out life with a memory loss. However, couldn't they have spread the tragedy around a bit more?
Sorry. There are several good reasons to like Mamoru. As I mentioned, he's loyal and trustworthy, as well as being extremely patient. He's caring and kind to everyone, gentlemanly towards his girlfriend's friends, extremely intellegent, and he looks good in satin. And he does go out of his way to make sure that no innocents are hurt in the crossfire, something certain other scouts don't even think about.