In Vanilla and TBC, the raiding endgame was based around the idea of progression. A guild did Molten Core first, and after they moved on to Blackwing Lair, then AQ40, and finally Naxxramas. This was the pattern guilds and players followed regardless of when they started. A new guild or raider was expected to start in Molten Core and move their way upwards.
In contrast, Wrath raiding was based around a different idea: focus on the current tier. In general, at any given time the community--including newer raiders and guilds--focused on the most recent tier. When TotC was released, that was what everyone did. When ICC came out, everyone went for ICC. You could essentially solo your way, gear-wise, to the entry point for the most recent tier.
Personally, I liked progression more. In a progression system we didn't beat bosses by drastically overgearing them. We didn't do much farming of outdated content. We did what suited our gear and personal experience levels.
But progression has a couple huge downsides. One is rerolling. If you want to change class, you start at the bottom and there's no quick way up to where you should be, based on experience. LK's solution was heroic automatically putting players at the second to highest tier (I'm counting raids, not sets, so 245 is second to highest since ToC is second to highest raid). I'd have preferred a system of retirement and hastened gearing and progression. Obviously this doesn't work quite right outside of a badge system, but the general concept is the same: trade one character for another.
Then there are the guild-killer bosses. These are the bosses that you get stuck on and do not get past. If you can't kill Vael, you're not going any further in Blackwing Lair and you're done on the 40 man track. Go back to MC, ZG, and AQ20, because you're not getting any further. Oh hey, three substantial raids even if you get stuck. Funny, that doesn't sound so bad after all. But no one likes to be stuck. The strange thing is that bosses in a raid don't follow perfect progression of difficulty, so Vael is harder than the bosses after him. If somehow Vael wasn't there, a guild could get a bit further along and possibly even kill all the rest and move on to the next raid. Or gear up some more (but not a ton more) and that would help. And then they can go on to the next raid.
So I propose a One Free Kill mechanic for raids. Each week you get one free boss dead. It gives no loot, no achievement, no quest progress, cannot be skinned, mined, or engineered. It's just not in your way anymore. Go on and die to something else. Note that this isn't one boss per raid, but per week, and it will be preserved by raid saves. By that I mean if you carry over a raid ID, you also carry over your kill limit. So you can't kill, save, kill next boss, save to next week, and so on. Also raid achievements would be tied to every boss, so skipping a boss will mean no raid achievement.
Obviously this hurts the model of progression. Even worse, it hurts some of that thrill of finally downing a boss. The raid itself will lose some meaning when the toughest fights can be skipped (or at least one). But I think it would be worth it for many players. Content isn't trivialized, just that the one particular fight that ruined everything cannot ruin everything.
My guild in vanilla actually was stuck on Razorgore, not Vael. So maybe we'd have not gotten anywhere anyway. But maybe having a different boss to wipe on would have helped. It's frustrating to do the same failure week after week. Diversifying the failure could help.
I think Cataclysm will add another mechanic to make progression raiding more playable: smaller raids. When a 40 man was banging its head against a wall, it was often hard to see exactly what was going wrong. Just recruiting was hard enough with so many people, getting the exact right players even harder. Dropping to a 25 man maximum helped. Making 10 man raiding a more significant progression path will do even more. It's a lot easier to troubleshoot a smaller group. It's easier to recruit and replace. Maybe 10-man Vael would have still help up guilds, but I think it would have killed a lot fewer.
But progression is dead.