Why on earth does everyone say copy/paste doesn't work? It works exactly as well as it does on Windows, which is to say perfectly well iff you're copying something vaguely textual and both apps are still running. Shut the source app down and what you get back might not look quite as good as it did before.
You know how many people I've heard complain about this behaviour, outside this thread, in my entire life? Zero. Nobody other than geeks even *notices* it.
(As for the video tearing, well, that's something else I've never seen, and is not something that wiring policy into the graphics server would do -- or, rather, its existence is merely a consequence of the sort of transition -- to composition -- which is pretty much entirely impossible to implement if you nail lots of policy into the graphics server.)