I don't know yet, we haven't managed to migrate yet, all I know is my coworker said it won't work for him. Will probably see for myself next week.
Anyway, they upgraded the pg gem in 4.0.1, that might be related.
I have been testing it out. It is cool, but if you are dependent on plugins, you will be pretty frustrated.
As for differences, the only thing I could find was this:
> This new version brings several improvements to the search engine (it's now much faster and includes new search options) and many new features: default issue status per tracker, multiple emails per user, ability to edit attachment descriptions and more. It now runs with the latest Rails 4.2.0 and ruby 1.8 is no longer supported. Redmine 3.0.0 requires ruby 1.9.3 or higher (and now supports ruby 2.2).