Well, men tend to outnumber women in such sites, so that's the start... also, people are only likely to respond (with interest) in the top 10-25% of people they communicate with based on looks... from there, personality comes into play.
It works both ways. It's just the majority of women don't respond to the majority of men, regardless of crafting of the message.. though I must say that I've gotten more thanks but no thanks type responses than actual interest, which is still less than 1 in 15 messages out for any kind of response.
Of course the buffet effect works both ways... people tend to see the options, and gravitate towards a level of attractiveness and features they like, which removes most of the pool on that alone.. now, if you aren't in that attractiveness block, your options for matches won't work as well.
I think that OK Cupid does better than most at matching similar attractiveness levels in terms of who you even see. If you happen to get any response from more attractive people (even a thanks, but no thanks) you'll see a shift in the matches coming up all around.
Then again, I'm a bit of an odd duck who tries to read the profile before seeing a picture too closely. So will usually respond if something catches my interest in the writing.. and unlike most men and women, I'm perfectly happy making new friends. Getting serious with someone I met about a year ago via Facebook... All dating site experience for me in the last 5 years has been pretty bad all around.
Would think it would be cool if you had a short (300-500 character) block to write about yourself, where you pick matches on picture and words... so you either see a blurb, or a picture... if there are two matches (either you like both, or both of you like one or the other) then you can communicate and see the full profile. Would probably be a more effective matching strategy at least.