Read this thoughtful article over at Splinksville postulating that multiplayer games might have a 4-year shelf-life. It's a very compelling argument, and draws examples from a long history of MUDs and other MMOs. A good read.
The constant escalation of the game's complexity and the solidification of its community very well may be the driving factor behind the loss of interest in WoW as WotLK cools down and stops being all new and shiny. I'd even run with this idea a little and say a big factor might be that it seems like the game's best days are behind it, and there isn't that much to look forward to. After Warcraft 3, everyone was itching to see more of Illidan and eventually take down Arthas. Those characters, along with Thrall, are what WC3's story was about. Any xpac after this will be about things that have had less anticipation built around them, and mostly come from lore built in WoW rather than lore built in WC3. Honestly, anything that happens in this game after Arthas's story ends is just a coda in preparation for the next game. It feels like we are riding out the next 6-10 months so we can be around for the climax, then many more will abandon ship.
Ulduar was late, and as I said last time, it's not enough content to fill out the game for another 6 months while we wait for the understaffed team to finish the next patch. My personal solution is just to do other things, but it's not going to help their business if a bunch of us decide to do other things all the time and stop paying. They need to announce what the next patch is about and start building interest in it in the coming weeks, even if they do so slowly, and they need to get that patch out before Blizzcon (ie in < 2 months). Then they need a massive announcement of the next xpac at Blizzcon and need to start building interest in that. If they don't I think that 4-year itch will combine with the lack of things to look forward to and lack of things to do in the game for veterans, and start sapping slowly but surely away at their playerbase.
The continued lack of alternatives also works in Blizzard's favor, especially with Champoins Online (SADLY!) pushed back to September. But if Blizz doesn't pick up the rate of release, they are going to find themselves seriously hurting when CO and APB come out later this year, and they'll have to fight hard if they don't want the KOTOR and Trek MMOs to make WoW a shadow of it's former self. It's leaking right now and there isn't even that much serious competition!