The 5.xx MoP Lua API changes will be coming in 5.0.4
I thought this one would be obvious but it caught me out. And I've been a regular in the addons community for a few years now.
There has been a metric ton of changes in the Lua API that addons use as well as changes in the macro system. There has been stuff removed , stuff added and things renamed. Certain things now work a completely different way.
So my biggest advice is start updating your addons and making a plan for when they break right now.
( Here is some advice for surviving itCollapse )