depends on where you go.
the beach resorts are nice and modern. be extra careful in pattaya though, ladyboys run that place; unless of course you're into that sort of thing (im not judging).
bangkok is a modern city but traffic is a nightmare. *tip - always make sure your taxi guy turns on his meter; if they don't they will try to scam much more money than the regular fare. dogs are everywhere. some parts of town are really run-down and shady. but dang, you will probably never see so many street vendors in your life. did i mention its hot? and humid?
not a lot to see in eastern thailand.
northern thailand is a little cooler, less developed and there are numerous hill tribes to see. lots of thai culture to experience in the north.
thais are generally good to foreigners. the only time you'll run into trouble with gangsters in thailand is drug-related stuff. if you're in thailand for other reasons than drug-related interactions, you don't need to worry about crime/gangsters.