One of the main ways to turn someone off is to come across as too sarcastic too soon. Sure, a little humor goes a long way, but sarcasm carries darker edges than your average, run-of-the-mill sense of humor.
When you're dating someone new, keep your radar tuned to how you're coming across, and watch out for letting too much sarcasm out of the bag. Sarcastic men and women love to tell themselves that sarcasm is a natural, harmless extension of their sense of humor, but the truth is that sarcasm is a defense mechanism. People develop this skill in order to make themselves feel more secure and powerful in interactions with others.
In short, feel free to let a little sarcasm show once you know someone well, but short of that, control your sarcasm in dating like the unruly child it is!