Biggest difference is the SMART TOOLS. And more AMA options. But the Smart Tools are a SIGNIFICANT change. The SELECT arrows are now in a different location, and that is unsettling to some. You can opt to use the Smart Tools, or not. But simply changing the location of the arrows throws so many editors...and many of them don't know you can turn off Smart Tools, that they REFUSE to upgrade.
I think there are some audio options too, on the timeline, that are different, but I haven't used MC4 in so long, I don't know what it's lacking.