There were a few issues, but nothing major:
1) The config API for my window manager, Awesome, changed a little. This is really just Awesome's fault, and was in an unmodfied part of my ~/.configs/awesome/rc.lua. The binding for mod-w changed from:
function () mymainmenu:show(true) endTo:
function () mymainmenu:show({keygrabber=true}) endSo I made that change in my config and things were golden. ~/.xsession-errors was helpful in finding this.
2) gnome-power-manager changed (either default configs or actions) so it hibernates my machine whenever I'm on battery power below 2%. Unfortunately, my battery-level reporting is broken and permanently stuck at 0%, so it triggers each time the power cord accidentally comes undone.
Fix is to use gconf-editor (or gconftool-2 or whatever) to set /apps/gnome-power-manager/actions/critical_battery to "nothing". Now it prints an angry message, but is otherwise harmless.
3) Not a new issue with 10.10, but something that's been intermittent with with Macbooks is a crackling on the left headphone channel. (The external speakers aren't good enough for me to tell whether it's on the speakers too.) The "S/PDIF" channel causes this -- mute it if it's not already muted, and the crackling goes away.
That's it!