Cambodia expat here.
If you are really heavily into archeology and ancient civilizations, then the old Khmer empire made way more impact than "just" Angkor Wat. Check the documentaries on the topic.
Cambodia is mainly a people-thing. You feel a lot of sympathy for the (mainly) air-head locals. It is a good place to be an expat 'cause of visa reasons. Also a good place to fuck boys and girls. Good place to break some minor laws and get away with it. Decent internet. It still feels a little bit outlaw, but of course not as wild as it used to be in the nineties (when I first should have gone).
Unfortunately you get a lot of the Pattaya scum filtering through Phnom Penh, but they are not as plentiful as in Pattaya or Bangkok.
There are more and more western hipsters being sighted, if this is a good or bad thing depends on you. You can get decent haircuts, get good coffee, hang in trendy bars, and even buy a locally customized cafe-racer.
I love Cambodia and can't wait to go back. Every time I go to Thailand I just want to get out. Also, the Thais suck the cock of the government. The Khmer love their king but not in a sissy submissive way like the Thais do.