Continuing the eternal series, part of the problem between casuals vs raiders is that the gulf between them is only one-way. Casuals cannot do raid content. However, raiders can do casual content.
In fact, they will do such content if the rewards are half decent, or if they are simply bored. So there's no such thing as content specifically for casuals. And this feels a little bit unfair to casual players. After all, both groups are paying the same amount of money to Blizzard. Yet the raiders get new content which they don't share, but casuals have to share all their content. It's possibly a bit irrational, but that's the way we are.
I think WoW needs to address this asymmetry (in addition to making it easier to raid). My suggestion would be an epic 5-man dungeon. This dungeon would be hard and would drop unique loot slightly better than Tier 1, but less than Tier 2 (a Dungeon 3 set). The dungeon would be on a 7-day timer, like the raid instances, allowing a small group to work on the dungeon over the course of the week.
The kicker, though, is that if you were saved to this 5-man, you would be locked out of the 40-man raids for the week. And if you were saved to MC/BWL/etc., you'd be locked out of this epic 5-man.
So for raiders, this dungeon presents a real drawback. If you go to this dungeon, you can't raid with your guild that week. If you raid with your guild, you can't try this dungeon. However, for a casual player, there is effectively no drawback to trying this 5-man, as you aren't raiding anyways.
The key is giving the casuals an experience that the raiders can't easily share. Raiders still have the lion's share of endgame and loot. However, casuals get a toy for them, and them alone.