Woman here who met my husband on OKCupid.
I only responded to messages that were personal and where I felt like I had something in common with the person sending me a message. My profile said I enjoy bad puns and football, so if someone sent me a bad joke or brought up how the football season was going, I was very likely to respond.
Things I didn't like/would cause me to not respond: the initial email was too long (about 2-3 sentences is plenty, just enough to see that he read my profile and that we have something in common), there were too many questions (it felt like I was being interrogated at trial), we clearly didn't have anything in common, he only sent me a message based on what I looked like, or he shit-talked my favorite football team.
My advice is to keep it simple. I was never one of those women who got 50 messages a day when dating online - I guess I'm not cute enough! - but I still definitely didn't respond to every message I got because it gets overwhelming...especially the very long emails and/or the ones that just said something to the extent of, "Great pic! What do you like to do in your free time?" (Uh, read my profile, dummy.)
Good luck!