Ummm yeah. Goodbye Decursive. So long Panza, Benecast, and CastParty. So long one-click heals of any kind. So long almost every UI addon that's required by most guilds for endgame raiding. According to the post, most of the addons you know and love won't be allowed after they run the patch on the expansion.
Now, there might be a little bit of leeway on this. Some "popular and benign UI mods" will be allowed to "take advantage of new functionality" being built into the expansion. So it's possible, depending on your reading, that mods like CT_raid (which really should have been built into the game in the first place, if you ask me) will be allowed to exist. But there's no telling at this point what this "new functionality" does. And since Blizzard doesn't elaborate on which mods cross the line-- is spamming Decursive their idea of cheating? How about clicking off debuffs person by person with CastParty?-- this little post doesn't really do much more than throw things up in the air.
And there's no telling where they will land. Expect an outcry from the UI community over the next week or so, or at least until they can figure out what's happening. If there's a way around it, this may not be as big as it seems. But if Blizzard is really serious about shutting down some of these addons, not only will raiding grow a whole lot harder overnight, but the addon community (which has done a lot of great, hard work for Blizzard, without pay) might be affected irrevocably.
[ Thanks, Owlboy! ]
Update: Blue is answering some questions in this official thread. Basically, any mod that lets you click one button to do different things (Decursive) is out. Whatever you're clicking is not allowed to choose a spell for you. Now, I have zero experience with UI coding, so if someone else can explain it better, please do. But the fact remains that Blizzard is cracking down on a segment of the UI community, and it's likely to change the way almost everybody operates, especially in a raiding environment.