I don't know, really. There's just something about his anime personality that grates on my last nerve. I mean, disregarding the fact that he's an idiot in matters not pertaining to his classes (ie. his love life), he was a complete jerk to Usagi when he first met her, and didn't really get that much better as their relationship progressed.
Now, I have nothing against Mamoru, in and of himself. Just the anime version. I actually rather liked his Live Action counterpart. In fact, aside from Minako-chan, Rei, and Artemis, he was my favourite character. Quite an improvement over the anime version, in my opinion. He actually seemed to be a positive influence in Usagi's life, whereas the anime version only seemed to cause her pain and grief as a result of his stupidity, not to mention his "I know what's best for you, so sit down and shut up" attitude toward her.
Just a personal opinion, though. Not to be taken as fact.
The thing I think most bothered me about anime Mamoru is that he was supposed to love Usagi. However, in nigh every episode, he would show up, toss a rose (A rose? What the heck? What did he do, spend hours whittling them from solid chunks of carbon-steel?), say a cheesy line, and disappear not to appear until his next oppurtinity to spout retarded lines that no one wants to hear. And, to top it off, he would show up when the senshi were getting their butts handed to them by some youma, do his "thing", and then disappear leaving them to fight the youma by themselves. The same youma that was handing them their butts in a sling two seconds before he popped on the scene. In the anime he was a completely useless character used only for the sake of his appearance in the manga, and occasionally further the show's plot. A role that could have been filled by any number of characters, aside from loyalty to the source material.
*sigh* Sorry, I'm ranting.
Moonlight Knight was still Mamoru, in most senses, but he was a lot less annoying, and a lot lighter on the cheese. Also, for being non-corporeal, if memory serves, he did a lot more than Mamoru did as Tuxedo Kamen up to that point. Even when TK was evil, he was still throwing his stupid roses, and spouting retarded lines. The only difference? They weren't "positive" messages anymore. Then, as usual, he'd disappear, never doing much.
As for Mamoru and MK appearing in the same scene together, let's just say Mamoru has so bad a case of schizophrenia that even other people see his delusions.
Chris