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.