Her name is reverberating around the PTR right now. In fact, within three seconds of logging on with my premade I was asked if I wanted to join a 10-man raid to kick her scaly butt. Of course I accepted and found myself once more outside the scary gates of the Broodmother's lair. Let's me be straight about one thing, the only things which have been changed about this fight are the suped-up uber-trashkin and Onyxia herself. There's no need for a key or attunement, you just grab a raid and walk right on in. The cavern leading into her lair is the same as it ever was, except the trash now have the power to kick serious butt against level 80's wearing full epics. Such is the case, our group wiped the first time we pulled two of the dragonkin.
The encounter itself runs exactly as you remember it, just with increased difficulty. The first time I ran Onyxia was the day the attunement was dropped and she wasn't exactly easy then. While the memory of the encounter hasn't faded, practical knowledge of her tactics is a different matter. Everyone in my group totally forgot about her legendary Flame Breath attack and we wiped on the first try. The second time, it was the dragon whelps. Trying to deal with two dozen whelps, a couple of dragonkin elites and a flying mother dragon ... well it's never going to end well.
Previously when I've done this encounter, the one thing which struck me was how bugged Onyxia was. She appeared to walk in mid-air without moving her legs or wings. She would sometimes stop for ten seconds as if lagged before unleashing vengeance. The refreshing part was that in this souped up encounter, this buggyness has been, for the most part, fixed. Not completely, of course, there was one fight in which she briefly froze but this is expected when you're playing on a PTR.
Perhaps the best thing is, there's nothing new. The tactics are just as you remember. She charges, you unleash hell. Eventually she takes wing, summoning a legion of whelps from eggs around her chamber or some dragonkin. As she flies back and forth, Onyxia spams fireballs and then does her Deep Breath attack, spewing fire that kills almost instantly. Then it's just a matter of rinse and repeat until phase three.
4880k health is definitely nothing to be sneered at. Onyxia is hard and we got her down to around 50% health several times before dying. The nice thing is the familiarity, rather than making raiders cocky, helps understand the fight but the increase in difficulty means this is definitely no longer a walk in the park.