Also, another idea: analytics. This is ripe for psychopathic manipulative people, so should be treated delicately, but for people who are just socially awkward, aspergers, or using non-native language this might be a huge boon.
After you send enough messages, you get some vague idea. Say, "people who answered questions x like this are likely to think you're obnoxious", "people don't even read your message — there's something wrong with your nick or the small version of your photo", "if you use the following words, the probability of answer goes up this much".
BTW, on OKC, I would totally love the ability to get analytics on questions I wrote.
Maybe the site could use its users as "captcha" solvers, in the form of rating/tagging content.
Ie the site pops up a random picture of another user occasionally, which the user must rate according to several dimensions ("photo quality", "objective attractiveness", "subjective attractiveness", "fashion impressiveness", "charisma", etc).
Or the site could pop up another user's profile and ask the user to rate it on interestingness, spelling & grammar, charmingness, etc.
Are you worried of people "obnoxiousing" someone of hatred out of spite? Well, there's two things, one is that only people you messaged can mark you as obnoxious, and only once. Two, hopefully after some tuning, becoming a "recognized pain-in-the-behind" would mean that a significant part of the site (or at least the part of it you're likely to message) considers you obnoxious.
I can easily see chicks confusing "ugghh that guy is a loser clicks obnoxious button" with "that guy sent me a string of genuinely obnoxious messages and actually deserves to be punished click".
First, I see using the word "chicks" as insanely offensive. That's what probably would get you marked as obnoxious, not the looks.
The second thing, getting marked as obnoxious means you're routinely messaging the wrong people. The only change I'd consider to combat this is the site warning you, that the kind of people you're messaging are obviously not that into it, along with any hints it could mine from the data.
Also, I consider your fear misguided. Trust me, the most common reasons for rejection of men by women on dating sites are not looks. The other way around, though...
How would you prevent abuse, though?