Reliable user experiences come from
a) Reducing degrees of freedom - remove the number of things the end user can tweak / change (the mac way)
b) Monopolistic fiat - Publishing standards and strong arming vendors into supporting those (the wintel way)
Linux tries to approximate reliability using a "many eyes make bugs shallow" approach. However this kind of reliability is stochastic. Think of it as a gaussian centered around the defaults that ship out of the box on hardware that is fairly commonplace. The more you push your end config away from that center the more likely it is that you will have a suboptimal experience.
I agree that in an ideal world installing a new WM would "Just Work", but we don't live in that world. Try installing a new WM on OS X or Windows and see how well things go. Not trashing those operating systems, just pointing out that GNU/Linux does, indeed, offer more choice. The fact that the choices are not ideally convenient to switch between is really a fairly minor issue.
What does that even mean? That I have to have a special twm distro? Or an fvwm2 distribution? tvtwm was always my favorite window manager, but I seem to recall it needed some patches to really be perfect. Do I have to find somebody to build me my own distribution that has those patches?
A Window Manager manages windows, like the name suggests; it doesn't manage networking or sound. And if it does, then the whole system has failed to produce decent engineering boundaries and we should revert a few releases to when things had the correct levels of abstraction.
Desktop environments are sometimes set up to handle sound or networking.
I agree that it's not a good design, but I don't think it's as ridiculous for a desktop environment to handle these things as it is for a window manager to do so.
I think it means 'use Slackware'.
I'd recommend Arch Linux. It just makes sense.