http://wdnaddons.com/ has the PTR UI code. I looked at the changes and nothing stand out as an important compatibility issue.
There are a few interesting tidbits in there. Blizzard removed most if not all reference to GetText() and introduced a new escape code "|4" for correctly choosing between singular and plural form. They also introduced strreplace as an API to speed up the chain of string.gsub that is occuring in the Combat Log.
You can find reference of a new in-game service too. "Refer-A-Friend", that looks like it will allow someone to grant level (and/or faction?) to another player.