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GitHub User: statique
I noticed 2 areas of improvement:
Rails 4 syntax is less verbose and more intuitive when writing queries.
Rails 4 has a heavy emphasis on security.
There are several more features covered in various blogs and podcasts.
To summarize, Rails 4 "feels" more mature.
I took a huge commercial app and was able to upgrade it from Rails 3.2.13 to Rails 4.0.0.rc1 in half a day. Having the test suite in place made the transition process go smoothly. There are a few popular gems that we use that are not Rails 4 ready so releasing this upgraded app to production will have to wait.