You teach Linux? Really?
Then I guess you know already that it doesn't matter what desktop environment the user chooses. Applications written for Gnome tend to work in KDE as well, and vice versa. Any halfway-competent distribution just grabs whatever dependencies it needs.
Even with that kitchen-sink approach, you're still probably going to end up with less than half of the bloat of your average Windows 7 install, so why not just install all the common window manager libraries and let the users and developers both have whatever they enjoy using the most?
As for which apps will work on what distro, well I can only speak for the occasional commercial game that is released "for Linux". I can assure you that Epic and Id's products have worked on every distribution I've tried them on. That said, I haven't tried installing Doom 3 on a Belkin router yet. Your mileage may vary.
IM server, what? I think if you're wanting to run an IM server, you're already getting way into techie territory and beyond what 99.9% of users will want to do. If it takes a little minor fiddling with a config file, so what? And would you not have the bewildering array of IM protocols under any operating system?
Standardising on one interface means you will piss off everyone who doesn't like that interface. Good luck getting a successful distribution going that supports Gnome but not KDE, or KDE but not Gnome. And getting every distribution to use the same kernel version? Did you not read that allegorical tale of the cathedral and the bazaar? Yeah, good luck.
So far, so yawn. You've come up with the same tired old arguments that were incorrect five years ago and even more incorrect now. Linux's biggest problem as a domestic OS is simply that old catch 22. No market = no developers = no market. Everything else is sorted, drivers included. Even that old wifi chestnut is fading fast, and any problems there are probably as much to do with the same catch 22 as anything else.
So yes. Written on my Ubuntu laptop. It's quite nice actually. And not shit-brown either. I know how to change background colours.