With Spaces+Exposé, I could do two buttons presses to quickly see every window in every space. Mission Control removes this ability and instead stacks windows, making it harder to find what I'm looking for. Snow Leopard would easily let me select an individual window in another space with one click, while Lion either requires me to slide several times, pick the window, and do an app-only Exposé, or select a space, use Mission Control again, then select a subwindow. It kills productivity.
Additionally, Mission Control is simply much more laggy-feeling. I could easily switch between apps and spaces with no hesitation in Snow Leopard. In Lion, for whatever reason, it was that decided it was important to eliminate this in order to do a fancy slide effect. Lion also gives you the ability to set a unique wallpaper for each desktop, which is nice for some people, but it forces my computer to refresh the items on my desktop each time I scroll into another space.
Mountain Lion has a checkbox to ungroup applications in Mission Control. It's basically like 10.5 Exposé with an added spaces bar at the top, and a background that shrinks for some reason.
I usually don't mind when Apple tries out a new direction. It seems several of their products have taken two steps back to (try to) make a big leap afterwards, like iMovie and Final Cut Pro X(?).
It's only terrible because Apple forced all users to their Revision A software. Axing MobileMe and iOS development tools before Mountain Lion cleaned the mess up wasn't a nice move.
Additionally, Mission Control is simply much more laggy-feeling. I could easily switch between apps and spaces with no hesitation in Snow Leopard. In Lion, for whatever reason, it was that decided it was important to eliminate this in order to do a fancy slide effect. Lion also gives you the ability to set a unique wallpaper for each desktop, which is nice for some people, but it forces my computer to refresh the items on my desktop each time I scroll into another space.