There's a lot of fairly useless information about this, mainly from deniers who insist that an in-place upgrade will never take more than an hour on any hardware with any amount of files. Sadly, not true.
Two reasons I found during a recent upgrade:
...
It seems that the "killer feature" of XCode 4.3, the new Storyboard
editor, doesn't like VNC. An important feature of this is the ability
to hold down control and then just drag objects around to form
associations, but over VNC, nothing happens. It's a...
Trying to use a Mac Mini over VNC... last time I used it, it was
fine. Tried today and got no keyboard response, even though the mouse
was fine. Hadn't been restarted, hadn't had any patches applied.
Well, to cut a very long and tedious story short,...