1.: Missing dungeon finder is not new-player-friendly. I know it wont be enabled before ICC is released and it might be never activated and I dont want to discuss if this is good or bad for communication, end-game-experience, etc., but for new players RDF would be nice.
2.: Many Vanilla and TBC dungeons arent working as intended, some quests and questchains are bugged. Warmane staff is looking into it and will fix those dungeons.
3.: TogC is not doable for many guilds. I like it the way it is, but let's face it: Ppl expect to achieve the latest content in a game.
While the idea of very hard content attracts many players, actually very hard content repels them. You can see that development in other games, too, like for example Diablo 3, where everybody was excited about a difficulty which was not meant to be doable, but when players faced the fact that they actually wont be able to get through this content, they qq-ed until blizzard eventually nerfed it.
With many guilds raiding ToC25 and only few Guilds being able to raid TogC25, there is only very little progression to me made for players in the later content, thus some are leaving (and will be back for ICC just to leave again when they learn that they wont be able to raid ICC hc25).
1+2 are the reasons why we dont get much new players and 3 is the reason why some old players leave.
Of the 12k players online (+2k on queue) many went to icecrown, obviously. But that's only the reason for the first sudden drop of population after IC opened it's doors. The constantly decreasing population is because of other reasons.
I hope warmane will look into the reasons why new players dont stay here for long and wont nerf the heroic modes of endgame raids.