Blizzard devs mentioned last week that--unlike previous expansions--there would be no new 5-man dungeons in Mists. However, there will be a few new scenarios. It is interesting to compare scenarios with dungeons. Why can Blizzard make new scenarios but not dungeons?
I think the key is rewards. One interesting thing about new scenarios is that the rewards are not better than old scenarios. Blizzard makes a new scenario, and throws in the group finder rotation. It doesn't obsolete the previous scenarios.
Indeed, scenarios have no intrinsic rewards, or rewards unique to a particular scenario. The only reward is what you get from the launcher, valor and the random loot bad. If Blizzard wants to improve rewards for scenarios, they improve all scenarios.
Meanwhile, a new tier of dungeons would be expected to have new and better loot. But this instantly obsoletes the older dungeons. We get into situations where we run the same 2 Zul dungeons over and over, or the same 3 Icecrown dungeons.
I wonder if the player base would accept a new dungeon with the same item level rewards as the previous dungeons. After all, we now have Looking for Raid to take care of gear progression for transient players. So a new dungeon would simply be new content, new achievements, and a new challenge mode.
But somehow I don't think the player base would be happy with this. The expectation is that new dungeons bring better gear. But simply obsoleting that much old content is not a good choice either. So maybe Blizzard's choice to not make new dungeons, but focus on scenarios and raids, is the "least-worst" solution.