gksudo -u gdm dbus-launch gnome-appearance-properties
Which is fine, I guess, except that it overwrites my theme/GTK settings for my openbox setup, and I have to configure it again, unless I use the exact same settings for GDM (which I ultimately decided on doing). So far that's my biggest problem with gnome 2.28. However, I won't be downgrading, since I have configured GDM for the time being and probably won't be changing it anymore. Another issue is that there are random crashes back to the GDM login screen since GDM starts on tty2 now, as it's the first available getty (when GDM starts), and when the tty2 refreshes, it then kicks you off gdm and has it restart on tty7, as it should actually do from the start. I've put a temporary stop to this by commenting out my tty2 getty entry in /etc/inittab, but I'm so used to my X being on tty7 that it's taking some getting used to. But again, I can live with it, I just see no reason why this new behaviour is there. I am also curious why they decided to not include a configuration tool for GDM itself, besides the usual gnome-appearance-manager, it would just make life easier.
Moving away from GDM, I don't really see too many tangible differences in Gnome 2.28, since I run Openbox, and hardly use any gnome apps besides GDM for logging in and shutting down, etc. I've noticed a few layout changes in the Gnome applications that I ended up using to get GDM to work, and they're nothing intrusive or random, and makes, if anything, the user interface more logical. I honestly don't expect there too be too much of an improvement on the interface level until 3.0 comes out, but I could be wrong. In any case, Gnome 2.28 seems to be a nice update for most Gnome-related applications. There are a few showstoppers with GDM (inability to use themes, changing themes results in overwriting theme settings for user account, starts on tty2, etc.). The tty2 problem could be solved by editing the custom.conf, but sadly they dropped support for the [servers] flag, which, again, seems illogical to me. Also, I use lxappearance in Openbox along with Nitrogen, so that the changes to your themes may not occur if you used gnome-appearance-manager to set them as well.
Hope that you find this opinion (it's not a proper review, it's just my impressions on it) useful,