Right. MAJOR. For example I use awesome 3.5 as my WM. It's a pain in the ass to install it on Debian. Wheezy libraries aren't compatible with the latest version of awesome. If you want to upgrade libc, you'll break the whole system.

It's not that simple. Ubuntu has a lot of patches to make stuff interact nicely, and installing a diferent DE/WM is not as trivial as you'd think.

The opposite is true in Arch: it's extremely simple to change de WM, and chances of breaking anything else are minimal. This is especially due to the fact that you build the system yourself, so you know exactly what changing the WM affects.