While Symfony 3 shares most of the same concepts as Symfony 2, the implementation has changed considerably. Here is what is turning to be the must fun part of the upgrade:
The documentation url includes the word current. Up until yesterday, current pointed to the Symfony 2 documentation. Now it points to Symfony 3. The two are definitely not backwards compatible. So now we have umpteen thousands of articles and help posts which are linking to incorrect documentation.
Lots and lots of questions already on stackoverflow. Going to be a fun ride.
Look here to see some of the recent changes: http://symfony.com/blog/category/living-on-the-edge
Mostly incremental at this point. In fact Symfony 2.8 and 3.0 are pretty much functionally identical.