I'd recommend you go through releases list https://world.episerver.com/documentation/Release-Notes/ (filtered by feature/ui feature) or https://world.episerver.com/features/ (as this is older versions) and breaking changes https://world.episerver.com/documentation/upgrading/Episerver-CMS/cms-11/breaking-changes-cms-11/ identifying the features that are applicable to the upgrade and your solution. As Tomas said, the implementation of the TinyMceEditor changes, I think off the top of my head there were some changes to the Dojo paths as well (dots to hyphens I think) but it's all detailed in the links.
Check for XhtmlString formating error in views => https://devblog.gosso.se/2017/11/uppgrading-to-episerver-cms-11-causing-errors-and-how-to-solve-them/
And, of course, the list of breaking changes for CMS 11.
We are upgrading from version 10 to version 11. I need to test the upgarde and looking for suggestions on which functionality to focus on. Did anyone observe any bugs after the upgrade?