People will represent themselves with misleading photos - years out of date. (When we met in person I was disturbed by the difference. I made the effort to have a good time anyway, and was awarded with a second invitation, which I had to politely decline, explaining why.)
People who are very good essayists given plenty of time at home turn out to be awful conversationalists in person.
The flood of responses compels a person to overbook and overextend themselves. Sensible people who would otherwise date only one person at a time, and with due hesitation, will instead start flipping through people like a card catalog, getting a bit drunk on the tension and the variety.
The same person can have a dull evening trading disinterested banter over loud music one week, and then have a warm, giddy exchange next week that sets the stage perfectly for an exciting relationship, and the only difference will be the MOOD of the subject. And, sadly, online dating IS NOT GOOD for one's mood.
Because of this I've discovered that the best way to find people who still have some sense of enthusiasm or sanity about them on the site, is to pay the most attention to people who have only recently signed up.
My standards are high, my time is valuable, but being the nice guy via online dating is exhausting.
I need to take it in small doses, with the filters turned up high and the expectations dialed down low.