Tier 11 was more of a "Traditional" Raid experience in that we had several unique areas to raid in and many interesting applications of mechanics to challenge players.
Tier 12 returns to more of an ICC format of several bosses with an established lore character as the final villain.
Tier 11 really spoiled the older WoW players who fondly remember the huge variety of locations to raid during BC and, to a lesser extent, Classic. We were given a glimpse of a return to form for WoW with a few nods to the Casual player that helps keep the game profitable.
Admittedly, Blizzard made some of the Heroic T11 content cripplingly difficult for a large quantity of players. Many of the encounters were absurdly unbalanced in a progressive point of view. For example; the Defense System on heroic in BWD was way more complex a fight than many of the later Heroic modes. To rectify this, they attempted to make T12 more open to players of a variety of skill levels.
Some would say they took it too far in one direction and didn't make an effort to maintain an excellent balance. I would personally tend to say that 7 bosses in a single raid is sort of short sighted as you can clear the normal version in a single night and then your pretty much stuck with nothing to do other than alt runs.
With the news from yesterday, we see that Blizz has decided to do 'something' about the Shared Lockouts. I tend to think that this means that, rather than providing more content per tier, we are likely to see Heroic and Normal modes have separated lockouts again. I know that some want there to be separations for 10 and 25 lockouts, but lets be realistic; the 10/25 lockout works toward their goal of not wanting to punish small Guilds or Large Guilds. Separating Heroic and Normal as two lockouts gives the more raid oriented Guilds additional content per week while allowing the Casual Raid Guilds to continue at their own pace with no real penalty.
But anyway, not to get too far off topic; Moreso than PvP, raids are a challenge to make perfect. Blizzard has made a major effort to put lore into their recent raids. It seems that they want to continue in this direction by combining player forces with Hero units. This automatically creates an expectation that the player will not do everything in a fight to make the difference. This was poorly shown in the LK fight and shown again (much more effectively) by the Assistance that the player receives in Heroic Ragnaros. This is a problem because you have three types of raiders; gear-seekers, lore-seekers and challenge seekers. There is a hybrid of the three, but that particular type of player is a rare extreme. Satisfying any of the three is, by the nature of their playstyles, damaging to the other player types.
I believe that what we are likely to see is a rotation on raids that follows something along the lines of challenge - Lore - Accessibility. No particular flow between each, this is just my idea of how they go about laying their raids out now and moving forward.