This feature makes really interesting reading, and GuildOx have gone to some length to ensure the data is accurate, saying that they have used their Character Database and filtered out alts using their alt detection.
WoW Insider could spend all day drawing interesting conclusions from the database, for example, the highest ranked holiday achievement title is The Hallowed, so that could mean the requirements make it the easiest to get, or that people just really love Hallow's End. In a similar vein, more players have the Arathi Basin Victory achievement and the Alterac Valley Victory achievement than have the Warsong Gulch Victory one. Why is that? Of course, this feature can't answer that question, but it's fun to speculate!
And while you're over at GuildOx, don't forget to check out their popular mounts and pets databases -- see where yours rank!