There's the small stuff like "class" is a keyword in C++ so not a valid variable name.
There's the fact that C has continued to evolve so there are new C features that haven't made it into C++ yet (VLAs).
There's stuff that has been implemented differently in both in mostly compatible but sometimes observably different ways (e.g. the types of character and boolean literals)
Thanks for the mention! I heard about Carbon years ago but I'm happy this time I could dig it further for insights now.
It's pretty fun to think about "Carbon to C++ is Kotlin to Java". One very important takeaway from all the discussions here is that, I cannot ship a language right to the target (small) community, as it's impossible to control how people decide to use this language. Which means, I have to focus much on how to improve the experience of application writing. Carbon would definitely be one of the inspiration.
Oh yeah, and I don't need to handle seemless integration with templates, I'm lucky.