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?
Test that TinyMCE still meets the requirements. You have probably moved the configuration to code.
Usually not a lot of problems.
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.