Pre-Burning Crusade, it was widely acknowledged that in general, the Alliance was superior in PVE and the Horde was superior in PVP. Aside from a few outstanding Horde guilds like Elitist Jerks and Nihilum that regularly got world firsts, the top 10 kill list of new bosses were usually dominated by Alliance guilds. Horde members constantly complained on the forums that the developers favored Alliance -- paladins got raidwide buffs, dwarf priests got fear ward, etc. Alliance fired back the same thing about shamans and War Stomp in PVP. But finally, in Burning Crusade, the Horde got paladins, the Alliance got shamans, and no one could really complain about anything. So what happened?
Well ... maybe the Horde did have something to complain about after all. Looking at WoWWiki's guild progression page, five Horde guilds and five Alliance guilds were in the top ten for killing Nightbane in Karazhan and Doom Lord Kazzak. But after that, things solidly shifted in favor of the Horde. Gruul: 6 out of 10 guilds are Horde. Magtheridon: 7 out of 10. Hydross the Unstable: 8 out of 10. The percentages shift, but Alliance has only caught up once, on Leotheras the Blind.
So what happened? Was the previous Alliance domination only a myth? I can't find any records of the top ten boss kills in AQ40 and Naxx, but I recall that there were far more Alliance than Horde at the time. I also recall many, many threads arguing about what advantages paladins had. The Horde gets paladins and the Alliance gets shamans, so everyone's equal, right? Apparently not. What mystical force is pushing the Horde ahead?
I asked a couple Horde stalwarts on my server, and they had some interesting ideas. Since the Hordeside is smaller than the Alliance on most servers, the Horde gets to know each other better as a community. We know who's good and who's bad, and guilds can recruit accordingly. And while the Horde who rerolled to paladins seemed to do it for PVE reasons -- "Hey, the guild needs a pally and I'd kind of like to heal for once" -- a lot of Alli shamans seem more PVP-focused. Or it could just be dumb luck that the Horde's ahead right now.
Why do you think Horde guilds have jumped to the fore of PVE post-Burning Crusade? Or is the whole thing just coincidence?