Judging by the comments in the "Aquarium" thread, folks have a lot to say about nice guys and the women who (do or don't) love them (enough).
No doubt there's some truth in perennial complaint that some types of niceness get short shrift in the dating market. Some people do seem to be drawn towards abusive partners. Who knows why? Past abuse, low self-esteem, bad judgment, personal idiosyncrasy, and bad luck probably explain a lot. Inequality is another major culprit--power imbalances invite abuse. Machismo is almost the antithesis of nice, and traditional femininity often requires women to indulge, ignore, or exalt men's bad behavior.
It's also true that the dating scene often rewards confidence, persistence, good looks, and conspicuous consumption over more substantial attributes. As John Emerson argued in the Aquarium thread, there might even be an inverse relationship between these qualities and niceness. I'd be curious to know if that's true. Thad suggested offline that some self-described nice guys may have difficulty recognizing the social nuances that make an approach seem charming rather than obnoxious. As a result they may be more reticent to approach women and more apt to perceive other men as being obnoxious.
However, guys who attribute their dating failures to niceness per se are often being self-serving. It's comforting to attribute to excessive niceness what might be better explained by shyness, awkwardness, or other less flattering interpretations. (I'm equally suspicious when Maureen Dowd complains that she can't get a date because she's too intimidating. Frankly, there are more parsimonious explanations.)
Often, the self-proclaimed nice guy wants special credit for just for being nice. It's as if he wants you to exclaim, "Oh, you poor fellow. What a burden it must be to treat women as you'd like to be treated. Above and beyond, old chap. Above and beyond!" I'm all for niceness, but I consider it a basic moral requirement for all humans, not a special bonus feature.
With certain notable exceptions, nice guys don't feel compelled to tell you how nice they are.* In my experience, most of the men who explicitly attribute their romantic failures to their own niceness are playing some sort of unendearing head game. Note, I'm not talking about acting nice, considering oneself to be nice, or valuing niceness in others. I'm talking about guys who tell you how nice they are and go on to complain about how women (read: you and your friends) don't appreciate nice guys (read: me). The subtext is that if women (you) weren't so stupid and hypocritical you'd appreciate nice guys (beg to blow me).
At worst, self-proclamations of niceness come across as vaguely menacing. The logical inference is that the speaker doesn't believe that women want to be treated well and that he might just drop the whole nice act. After all, if he thinks women like being treated badly, he might feel entitled to give them what he thinks they want.
*No offense to present company. Internet discourse is different from face-to-face interactions. We all have to describe ourselves a little more explicitly in a written medium. Maybe self-described Internet nice guys are unfairly getting tarred with the same brush as the guys who feel the need to go on about their niceness in real life.