Background: I am a plugin author. Our plugin needed work to make it compatible with 3.5, which we did as soon as we discovered a problem. However we are still dealing with users being locked out of their sites after upgrading to WP 3.5, without upgrading our plugin beforehand. It seems that 3.5 cost a lot of grief for users; maybe this was no worse than earlier releases, but one would hope it was a one-off.
I think there is much that can be done to avoid such situations in future, but from browsing the "make" forums I don't see any recognition of this as an issue. I suggest:
1. WordPress upgrade automatically disables plugins or themes not marked as compatible with the new release (or upgrades to a release that is marked as compatible)
2. WordPress addresses the problem where display of warning or notice messages can actually cause loss of display/function (Warning: Cannot modify header information).
3. WP provides a decent error-trapping mechanism that works across plugins and themes, and can include e.g. auto-deactivation of plugins causing warnings on web-page display or leading to loss of function.
4. (Perhaps) Upgrade process (of both plugins/themes and WP itself) has an auto-restore function to previous release if after a period of time user has not logged in and ticked a box to say he is happy.
I realise that the makers of WP are volunteers and it is a community project and I thank them for creating it, but in my experience most of today's WordPress users are not developers, and if WPs success is to continue we should recognise this.