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For purposes of clear discussion, Gated Progression in this context refers to the need to clear Raid X to access Raid Y to access Raid Z, and ignores any and all attunements.
Many people herald the success of TBC rather blindly, forgetting the issues this caused, constantly.
Not attuned to Black Temple? Well, your guild has two choices come raid night. Not bring you, or go back and clear out SSC/TK. Not attuned to Hyjal? Same choices.
Getting a group to do things like Gruul and Magtheridon when everyone else was in Sunwell? Yeah, good luck.
This lead to many guilds 'poaching' other guilds of talent, which just made progressing on a boss in a lower tier all the harder. You think you have trouble right now replacing a tank who goes missing? Back then, your tank would just be gone, off with another guild further progressed than yours. If he's not a jerk, he might be willing to come and walk you through something next week or so, but most didn't return.
In before "but that never happened to me personally, [insert anecdotes about friends]"
Trust me, it killed guilds.
Also, for the people who complain that they can't find a group to do T11 and T12 right now but have killed Deathwing?
Imagine the inverse for a moment. You're stuck in T11 and seeing Deathwing is pretty much an impossibility, barring a miracle, or a carry from another guild. Except, that guild would have to carry you through T11 AND T12 before you could ever set foot in T13.
This also makes it remarkably difficult to involve things like alts. It's a pain in the butt enough to gear up through multiple tiers of content, never mind doing so with an alt, never mind if there are barriers such as gated progression to deal with.
Yes, right now you can go and clear 5 man heroics then LFR and kill Deathwing. In the old world you could toil in 5 man heroics for ever, maybe clear Karazhan if people were still running it. Which, by the time Sunwell rolled around, finding a Karazhan group was a feat unto itself. Which is why statistically, very few people saw anything past Gruuls and Magtheridon. 10% of players saw SSC or TK, 1% even set foot into Black Temple, let alone Sunwell.
Currently, 11% of Guilds (not players, guilds, out of more than 60 thousand guilds tracked) have managed to down Heroic Deathwing. Just short of 6% have the Meta Achievement completed. That is a huge improvement over the Burning Crusade's Gated Progression model. And equally successful was the non-gated system used in Wrath which saw much higher numbers of players downing the Lich King and Ruby Sanctum and a much larger raiding playerbase than TBC could ever have dreamed of.
As the above rant relates to Mists of Pandaria, where there is some small amount of Gated Progression in that you have to clear Terrace in order to access Heart of Fear, but the difference is twofold. The Gate in this case is a skill gap, not a gear gap. Heart of Fear comes with the advertisement right on the box, this raid is harder than Terrace. It's also only gated (so far as we know) between two raids on the same Tier, not two raids in two different Tiers. IE-Tier 14 clear will (likely) not be a requirement to access Tier 15. And even then, it isn't going to require a quest from one to access another, much like the fiasco of Keys for Eye of Eternity back in Tier 7.
The negative I see here is when people do eventually hit that skill gap, but so far it isn't looking too severe, and the testing is looking really sharp for normal mode. In LFR I imagine that the ilevel minimum to enter Terrace will be lower than the ilevel to enter Heart of Fear, or it might be the same item level. If there is a difference, it won't be much, and there are more ways to improve your gear than ever before in MoP.
Just some thoughts.