Favorite thing Thai girls in general are kind, polite and extremely decent, however as we all know there are exceptions. There seems to be a division between the more traditional girls and girls that often hang out in bars. The kind of girl you meet totally depends on the bar you go to, the "bad girls" all hang out in one of the three sex area's being either Nana area, Patpong or Soi Cowboy. Don't expect to meet your dream girl here but if you're looking for some quick action these are the places to be.
there are the intermediate bars where you really don't know what the girls are up to, your money, free drinks or they really like you? A good measurement tool to find out what their intention is is to calculate the Thai girls to foreign guys ratio. The less foreigners the more decent the girls are probably, also pay attention to the number of Thai guys, no Thai guys means bad girls who are after foreigners.
If you're looking for a more peaceful evening without being harassed go to the bars outside the main tourist area's with mostly Thai customers. People are still extremely friendly but they're just nice because Thai people are really kind, not because of the money.