Red exclamation mark when trying to update Nexus 7.
I had this problem because I manually (via titanium backup) uninstalled one of the system apps: the korean keyboard (which, for me, is rather pointless). Somehow that caused the upgrading process to fail, maybe because my system was not "intact". After I restored the app it worked flawlessly. Did you make any modifications like that? If so, try reverting them.