One thing I encountered during my Loremaster-ing and which I hadn't taken notice of in a while were the elemental invasions. Have you ever seen one? According to WoWWiki they are quite an old feature (added in patch 1.4) but personally I didn't actually take note of them until my fourth alt or so noticed an odd yell in Winterspring one day.
Basically what happens is that every couple of days or so, a named level 58-60 elite elemental spawns in one of four zones on Kalimdor (earth in Azshara, water in Winterspring, fire in Un'goro and air in Silithus), surrounded by a lot of non-elite mobs of the same type (called invaders), and then patrols the area while yelling the occasional taunt at the zone in general. They are pretty tough at level, but if you outlevel them by a significant margin they quickly become trivial, just like normal mobs.
Nonetheless I still love looking for them whenever I see the familiar signs: It's like finding an exceptionally rare spawn. The loot is pretty damn good as well, they all tend to drop five or six items at once, usually a bunch of crafting materials of their respective element (elemental/essence of earth/water/fire/air) and some BoE gear, which still adds up to a couple of gold. Talk about loot pinatas.
I've killed Princess Tempestria multiple times by now, Avalanchion and the Windreaver once each, but I don't recall ever catching Baron Charr even though I've seen his invaders around - I guess someone just beat me to the punch each time. There may not be an achievement for collecting elemental kills like this, but it still feels like one to me.
With the upcoming expansion paying more attention to the elemental plane, I wonder if these elemental lords will make a comeback as serious challengers at the level cap? It would certainly fit with the overall theme, and if they were hard enough so people would actually have to team up to kill them... ahh, the possibilities. I'll try not to get my hopes up though, as I wouldn't be surprised if even Blizzard themselves had kind of forgot about these guys at this point.