You are those people, so I'll tl;dr them for you here so you can check it out if you need to make adjustments to your code and products:
- Backbone and Underscore updates in the core package — this includes some breaking changes in those libraries.
- If you interact with the “term edit” screen at all, see the changes that have some incompatibility issues with folks extending it.
- Selective Refresh should be enabled in your customizer work for a much better experience.
- Theme developer? Get an edge with custom logo support out of the gate with the 4.5 release.
There are obviously many more changes are in 4.5, but these may require (or would benefit from) code changes in your stuff. Get on it!