This is from personal experience. Note that it doesn't apply to anyone who might read this (except #1). I'm gonna mention ladies here, since that is my preference, but guys are the same. I'm not trying to be sexist.
10 - Most people expect more than they should. Nobody's perfect, gals... look for something good and don't let go when you find it.
9 - If you put up a personal ad, some people won't bother actually reading it. They'll then get upset with you when something comes up that you mentioned in the ad.
8 - Some of the folks you meet, while appearing to be perfectly normal, will turn out to be bat-shit insane.
7 - Some ladies are really 80 year old indian scammers.
6 - Most of the people you talk to, you won't hear from again. Unless you make an effort to stay in touch, which may lead to coming off as a creep.
5 - Again, nobody's perfect. If something seems too good to be true, it might just be.
4 - Spam. You will get spam.
3 - Sometimes you actually meet decent people.
2 - Everyone has secrets.
1 - Don't fall in love. Once you do it's nearly impossible to break out of it.
Anyone have their own to add?