Also, Gnome seems to require PCs with at least 128 MBs when used with lightweight distros (e.g. on Arch, Gentoo, Slackware you will need a minimum of 110 MBs for a "clean" Gnome desktop, and after loading Firefox or Evolution heavy swapping starts) and at least 192 MBs on more popular distros like Fedora, Mandrake or SuSE.
Windows XP runs better than Gnome with 128 MBs of RAM (requires about 85 MBs for its "clean" desktop) and this means that a lot of cheap PCs, or even business/school PCs everywhere in the world can not run a Gnome desktop effectively or satisfiably. KDE's memory usage is smaller than Gnome's too (on my Arch Linux is about 95 MBs).
Some Pango/GTK+ speed optimizations would be nice too.