People of Cambodia are friendly but it was difficult for me to make any friends.  Perhaps what happened during the Khmer Rouge regime has scared the mentality of most people a little too much...

Siem Riep is full of Japanese for some reason.  Almost all the tour guides speak fluent Japanese!  I wish I could do that.

Most roads are unpaved within Cambodia.  Therefore, travel during the raining seasons is an adventure all in itself.  During the dry season, watch out for the resulting dust that is generated with each passing car or breeze.

Don't exchange too much local currency as USD is the second, if not the preferred currency.

Landmine is still an issue within Cambodia so be careful where you go and step. 

Children will often ask for money or candy before allowing you to take pictures.

When traveling within Angkor Wat, local "guides" will often give you a tour with or without your consent so if you don't want to pay them, it is best to ignore them as they can be extremely persistent.

When visiting Angkor Wat, you can go there after 4PM and the entrance is free.