As I understand it, the license changed the original BSD license "advertising clause" into a "credit clause", and required attribution be included in documentation shipped in a binary distribution. Many projects/products dropped XFree86, not only because of this special requirement, but also because X developers were jumping ship. Those who dropped XFree86 at 4.4 included Mandrake, Gentoo, Debian, RedHat, and OpenBSD. Here's Theo's announcement in misc@:
http://marc.info/?l=openbsd-misc&m=107696705911864&w=2
Be warned: there's 30 or so responses in follow-on discussion.
My understanding is the currently preferred ISC license does not have an advertising clause at all. Less restriction, rather than more restriction.