A Fountain Of Hatred
As you may or may have not noticed, there are a lot of BSSM fans out there that really love to hate Mamoru. Exactly why is beyond me, in all sincerity. I have thoroughly loved this character since I first laid eyes on him; which was the DiC dub in 1997. I thought I'd go over some general reasons Anti-Mamoists usually give, which I'll follow up with my logical, unbiased (pfft) rebuttal.
1. In the first season, Mamoru is really mean to Usagi.
...And Usagi was just a sweet little innocent angel, right? They both tossed some pretty hrsh exchanges back and forth, so you'd have to give this reason equal validity to Usagi if this is a reason you'd use. It is, and always has been, my firm belief that Mamoru teases Usagi because he likes her. Little boys are often mean to the girls they like. Since Mamoru has next to no real social skills, he's still stuck in this kiddish way of approaching his inner feelings. Usagi reciprocates by being extremely jealous when she finds out Rei and Mamoru are going out on a date. Although he covers it fairly well, I have always believed that Mamoru really does enjoy his encounters bumping into Usagi all the time.
2. He's stiff and no fun.
Mamoru is not a character to just take at face value. He's one you have to look beyond the exterior; work on a little bit. He was brought up in an orphanage where he was probably taught to be the ideal upstanding Japanese citizen - polite, quiet, and self-contained. He's far less likely to run around doing goofy things like Usagi, but I don't think that means that he is incapable of doing so. When he is with Usagi, he seems to be able to let loose a lot more. I've seen lots of instances of Mamoru doing something funny (in his own special way, of course) or making funny faces and whatnot. Just because he isn't always outwardly zany doesn't mean he can't be funny or have fun.
3. He broke up with Usagi once and didn't tell her why.
He didn't break up with Usagi because he wanted to; he thought that he was doing it for her personal safety. Check out The Infamous Breakup.
4. He doesn't have special powers like the other senshi. He's so useless!
This is a really stupid reason, I have to say. Do you have super powers? Didn't think so. So should we all hate you and call you useless? Of course not! Besides that, Mamoru does have powers. He has psychometry, and in the manga he is the carrier of the Golden Crystal; a gem which can be compared in power to the ginzuishou. Plus, in the manga, he does get an actual attack - Tuxedo La Smoking Bomber.
5. He doesn't DO anything!
Yeah! He only risks his life on a regular basis to protect his loved ones, acts as a confidante to many, uses his intelligence to gather valuable information for the sailor senshi, and is most of the reason why the city of Crystal Tokyo stands in the future. You'd think the guy would at least try and make himself useful now and then, geez!