[phaemon]

Well, it's difficult to explain why something isn't antiquated, but on the points you raised:

1. You misunderstood the full-screen thing. X11 does have proper full screen. The trouble is that it's a bit too proper: the full screen app receives all the keyboard instructions, so how do you switch to a different app? The app needs to release some keys to you, or you need to exit or kill it. This isn't so good for games where you might have a chat program running in the background.

2. The client-server architecture is not really relevant since it uses sockets when it's on localhost.

3. Ugly hacks? Anything more specific?

X11 is a modular architecture and the very fact it's still in use suggests it's an effective one. Look at the difficulty Microsoft had getting compositing working (XP -> Vista), but it was added to X11 without having to invent a brand new windowing system.

I'm not quite as passionate about X11 as you might think. It's not so long ago that I was arguing that I'm excited to see what Wayland and Mir can produce. But it's not because X11 is bad, it's because I think things can be even better.