Let's talk about testing.
Now, most [Ff]ree software in this world is tested in a "ship it to the users and see if they break it" kind of way, assuming of course the developers don't just adopt a "it compiles, ship/commit it" methodology.
This produces the kind of stability we've come to know and love. There's a cute KDE Krash Wizard thing. GNOME has something similar - Bug Buddy, I think it's called.
Every option, every checkbox and every slider produces a codepath that needs testing - and not just unit testing, but integration testing. KDE's configurability has brought it to the point of essential untestability - and this is borne out by observing the number of times the KDE Krash Kollektor pops up - in my recent (KDE 3.4 for a week) experiment, I saw it daily, reporting Krashes for all sorts of things; I mean, the panel (inexplicably called "kicker") had a very wobbly afternoon.
If you're an extremist, every configuration option represents a design decision that was shirked by the application designer.
Take a simple thing. OS X buttons. You can make them Aqua (pretty colours) or Graphite (grey). Why does Graphite exist? Is this a design decision that is shirked? Well, sort of. Graphite exists because you can't have spangly buttons in spangly colours if you're trying to do subtle colour work (if you're a professional designer or artist).
The purist in me says "Well, make them graphite all the time, then, and never mind having spangly things. Don't you people have work to do?"
The realist says "OK, give people the choice, but keep it simple; spangly or not-spangly. This provides the sensible limit on what we've got to test."
In KDE, I can not only change my buttons, but their colour, contrast, style, window style. And in the window style, I can move my buttons around (depending on whether the window style them supports this). With my software testing hat on, there's corner cases and whatnot all over this, but you can bet your arse it's been tested on the "well, no-one's filed a bug on it so it must be perfick" approach.
(The snark in me observes that of all the themes and styles and stuff shipped with KDE, there's not a single one that doesn't look like arse, but that's just me).
We've talked an awful lot about usability, but software that stays up is more usable than software that's just shown you the KDE Krash Kollector or the GNOME buG Buddy.
GNOME 2.10 crashes less than KDE on the same linux distro (Ubuntu) than KDE 3.4.