I have a friend who, due to a stroke, cannot read fast or skim over lots of text. It turns into visual noise. The same is true to a lesser extent with lists of symbols. So if the only way to select something is to search through every time (because the order keeps changing), that's a bigger overhead for her than for most people.
Keyboard search is also not that helpful: The stroke means they have to type with one hand, and can only manage a little typing, and it's difficult both with movement and with finding the keys. If they have to watch the screen at the same time as typing just to know when the search has narrowed to the right thing, that's double overhead with an extra helping for multitasking.
For that reason I've told them to avoid upgrading their laptop for now, until we see how Gnome 3 or Unity pan out for accessibility, and the comments on articles like this one don't have so many reasons to avoid for the moment - or until we've tested and are sure KDE/XFCE/LXDE are suitable for their physical needs.
For perspective, they cannot use a touch tablet or touch-screen smartphone either - those things require several types of fine motor skills and coordination that most of us take for granted Whereas the Nokia E63, with it's BlackBerry-style keyboard and tactile key caps, is surprisingly well suited to my friend.
I'm getting the impression, from various articles and comments, that Gnome 3 is not really designed for people who have difficulties like those described above, and does not (yet) have settings to make it more suited, which surprised me because older Gnomes seem to have put some effort into accessibility for different user needs.