The only items I move from an old system are:
- Fonts. Fonts can be costly and are required when called upon by a client to edit a previous piece.
- Client/Billing app database
- Email database
- A notes app and it's database (contains important passwords, registration numbers, etc.)
Beyond that, I install the OS fresh. I install all apps fresh and do not move any app preferences or custom items. As @DA01 commented, this encourages new directions and exploration. Not only giving the OS a fresh start but hopefully infusing the work with some of that freshy goodness.
Custom brushes, presets, workspaces, etc can all be recreated if desired. And in recreating items I may do a better job or find a different "tweak" that aides in workflow.
It should be noted that I always run a file/OS separation. My apps and OS are on one drive and everything else is on other drives. So there's never a need to move actual design files anywhere.
(Small rant: Be aware that with Adobe's CC subscription it means you need to re-download all apps. S0, for me, that's 12GB of download. It's not anywhere near as easy and nice as pulling out a DVD and installing apps in 30 minutes. And, you need to download again each time you need to reinstall an Adobe app. The subscription is horrible in this respect -- along with a few other reasons.)