Look at the interesting info of Aoc Gold Cataclysm Raid Design and they have learned from what worked and did not work in Wrath in this forum thread.
The developers believe that current raid design (separate 10/25 man lockouts) effectively asks raiders to raid twice as much if they want to maximize their emblem gain, which helps cause player burnout and boredom.
The ability to kill a boss in separate 10/25 man lockouts is 'nothing sacred in terms of changing the design, since before Wrath it was not even possible.
Current design accounts for maximum emblem gain per week, because many players will attempt to gain the maximum of emblems per week. Changing the raid lockout means that this design can be altered.
ToC four lockouts were far too many.
The designers expect more gearing up from quests and heroics (as well as crafted items) before people start raiding in Cataclysm.
The feeling is that the burnout caused 'in some of our most dedicated raider from feeling forced to run both 10/25 man content every week to stay competitive is a compelling reason to change the design away from allowing that.