Now that 1.5 is here(ish), it's time for the scary part... finding out what is broken with our themes and plugins, figuring out how to transfer our core hacks to the new version, etc.
So, seems proper to create a thread for problems with specific plugins. Gideon already promised to try to be as backward compatible as possible; I've got 1.5 beta working now, and copied over the badge plugin - so far so good, the badge widget shows up fine, admin/plugins works, manual badge reawarding works... still have to test automatic badge awarding, but everything looks great - thanks again, Gideon :)
Plugins that I won't be updating:
close question plugin
comment collapse plugin
ajax comment plugin
may be others, will have to see one by one how they work with the new code. But this would be a good place to help find answers to the question of which plugins are incompatible.