The animation system, the exposure of nameplates (though they just restricted that back down in 7.2 so you can't edit friendly nameplates in dungeons), macro conditionals (you've probably used these in macros, but you can use them in custom buttons/macros as well), .... I'm sure there have been others, but they're not coming to me off the top of my head.
Plus, the version of Lua used was upgraded to 5.1.
Basically, just start from scratch with one of your projects. If you need help, ask.