I think one of the most powerful improvements that is also a bit of a challenge for users are the feature improvements around the activity stream. Jive 5.0 solves some key issues that you have in 4.5.x such as no easy way to stay up on groups that you don't want email alerts on (a casual following of a group). The notification icon(shown on the right) is also great to allow you to visually see (the little number) when you have key activity that would be important to you (when people @ mention you or respond to content you created). The ability to "work the inbox" is also a nice feature that you never had in 4.5.x where you can mark an activity item as read and not have to remember which items you reviewed and which ones you didn't (image to the left). These features are very helpful but I have found are not as intuitive to the common user as we might have thought. So it is worth the time to put some training material (quick video overview) or one page sheets to explain these features.
Along those lines explaining the difference between "following" versus "tracking in my communications" versus "email alerting" versus "Joining a group". There is a lot of flexibility here, but with that can come some confusion as to when you should do which one ("follow when interested, track when responsible" was one adage or sound byte that helped me remember). But these features are great once you get used to how to leverage them, so it is worth the effort to try to explain them.
Good luck in the upgrade!