You can do this in most desktop managers for Linux as well, where Spaces fails is how each workspace in managed. In Gnome each workspace acts almost like a new lite session, for example only the applications in the current workspace show up in my task bar (this is optional by the way) and when I alt+tab then only the applications on the current workspace are tabbed through. This works very well because when I switch between different task (say photo editing and programming) I don't want to generally think about what other non-related task/applications are running on my system. In Spaces every workspace is part of a kind of global workspace and there is no hiding of task environments.