I consider myself a bit of a power user of my Macs. On an average day I have a dozen or so programs open at once and I like to organize them very particularly. In addition, I have a pretty decent sized external monitor that I use at work to enable me to see more and work on things without task-switching so much. For example, I might have Aqua Data Studio open on one monitor, showing me the DB schema, and CFBuilder open on the other, writing code. Sure, I could still do the same work using a lot of Cmd+Tab to switch back and forth between the two, but that switching comes at a time cost, and a mental context switching cost -- and then what is this giant external monitor good for?
Spaces was awesome. Exposé was awesome. And I get what they're trying to do with Mission Control: combine the two. Less is more. And it could have been awesome, if it weren't for a few big mistakes.
As of Snow Leopard, when you hit your Spaces hotkey (F8), you would see all spaces given the same amount of screen space -- now with Mission Control spaces you weren't already looking at are tiny, making it almost impossible to see what's open on them. Um, hello? I'm here because I want to see what's on other spaces. How about you give them more than 100px?
Additionally, when you would combine Spaces and Exposé (press F8 and then F9), the windows on each space would do the normal Exposé behavior within the screen real estate allocated to that space. What was wrong with this? This was powerful. This was beautiful. This was perfect.
The only conceivable problem they were trying to solve here, in my opinion, was that all spaces were pushed onto the primary screen, and the secondary screen would just be black. Sure, that's some wasted screen space, but it worked well.
So now, with Mission Control, you can drag windows or even a full application-stack (drag the icon, not a window, and it will bring all of that application's windows) to another desktop... BUT only on the same screen. So if I want to move an application from the left screen of space 1 to the right screen of space 7, I have to first move it to the right screen of space 1 and then open mission control and move it to space 7. (Or move spaces then screens, order doesn't matter. The problem is that it can't be done in one swoop.) This sucks. Not only does it add additional work and take away existing functionality, I was so used to the way it used to work that even now, months after Lion's release, I still fumble around a little bit when I want to do this. It still aggravates me to no end.
If I could disable mission control and go back to Spaces + Exposé, I would in a heartbeat.