To prepare for raids, I read the spell abilities on wowpedia, and THEN I read strategies there. Then, I hop over to watch a Tankspot video on youtube and read the comments on tankspot.
Boy, that sounds like fun, doesn't it?
For raids, okay, I'll allow the point. This isn't that crazy a set of research prereqs. But for heroic dungeons? I really still think the gap between PUGging normals and hitting heroics is too large, from the gear needed to the amount of gaming knowledge before the fact (see "spoiler" comment from Lissanna below). Making a stupid Dungeon Journal is a real coward's fix and, if what Restokin says is true, only a partially successful way to get players up to speed.
Third party sites and addons did the same thing, but better and with more detail.
That's just a sad state of affairs. The game should prepare you for bosses. There should be preparation quests. NPCs helping teach you what's going to go down.
That you're in a state of affairs where...
Most raid groups “spoiler” bosses before they step foot in a raid dungeon
... tells me Blizzard really hasn't finished the game. And with 4.1 only bringing serious new content for those done with heroics (the mini quest line was fun, but really only a distraction. And much MUCH too easy. It was like a 4th grade in-class worksheet), the gap isn't closing and the game isn't approaching finished at all.
I wish we knew why subscription numbers are down. I've seen hard core bloggers say it's because hard core players have finished the content. But you know, I really don't think 10% of the player population is necessarily hard-core, end game finished players. The drop is from the casual player too. It's really no surprise. If I hadn't accidentally forgotten to cancel my autopay, I would probably be taking a week or two break this month myself. I barely have time for Tol Barad (where I've painfully found that I won't yet have the rep to buy Stump of Time when I have the commendations), often squeeze in a few minutes on my underpowered laptop without the Razer Naga (really hurts the DPS), and often just log in for another frostsaber cub whisker. Dude is getting big though. ;^) Watch him shoot up around 0:14...