Also be wary of your projects Gems, we just finished a 3.2 to 4.0 upgrade and though rails itself was pretty easy, there were quite a few problems with things such as Solr, Formtastic plugins (some FormTag internals changed in rails4, which broke some stuff).

So yea, the biggest problem was upgrading to Rails4 and getting the Gems to work- many of which had large structural changes in the Rails4 versions. For instance, Devise changed how they handle tokens in the Mailers, etc.

We have a very large App and it took us about 2 months part-time work to migrate fully to Rails4 - of that I'd say about 70-75% of the work was a combination of rewriting strong_parameters/attr_accessible stuff and updating/monkey patching Gems to get them working. But we did go really far, we removed all the stuff that generated deprecated warnings and removed all the "migration helper gems" like "protected_attributes". So if your project doesnt depend on many gems it could be really easy especially if you dont care about using the helper gems like protected_attributes until you get around to rewriting things.