"We have structured the game in such a way that you can find a difficulty level you are comfortable with. Naxxramas is one of the easiest raids we have ever done. Malygos gets a little more difficult, and Sartharion with multiple drakes is fairly difficult. Going into the future we will keep adding even more challenging and hardcore encounters while still making sure players who just aren't into that can still enter raids. The game has difficulty levels. If you are finding things so difficult that they aren't fun then by definition you probably aren't a cutting-edge raider. That's cool. We want to make sure you still have plenty of interesting things you can do, even within a raiding environment" - GC Blizzard Designer.
I should go do a little research. I wonder how many people have twilight defender titles at he moment. I really suspect it is less than 5% of the wow players.
The server I am on is a pretty damn competitive realm. We had many more guilds in Sunwell/finished BT than more realms (we were, I believe, in the top 10 for numbers of guilds pushing Sunwell at the release of 3.o) So not only are we pushing content as a guild (my guild got Illidan to 9% the Monday night before the 3.o patch and ended up 3 bosses through sunwell by WotLK) - we feel middling because we have so many high performers on this realm, but actually, we are really quote up there in terms of progression.
Blizz will be doing more with raiding achievements in future, you will see. I honestly think they will be putting loot and attunement on them at some point (ie unlocking optional hardcore modes and bosses in instances, more timed runs etc etc) So you can still clear everything at a casual easy level, but if you want a challenge you need to get the achievements done.
And soon we will have a culture of not downing bosses, but getting achievements - everyone can see the content and the game will be puggable and accessible to the masses of casual players, Blizzards main player base.
And personally, I think that will be awesome. It will mean that we have a much larger pool to pick from as a serious raiding guild. People who started late are not at a disadvantage (think.... how on EARTH are blizz going to encourage new sales mid expansion when everyone is geared? I suspect that is actually a serious problem for them) and we wont have to put in a massive investment in time to gear someone up only to find they are epic fail. Gearing is secondary now, still a goal and still a driver to play, but it wont be the roadblock it used to be, I feel.