C# is actually a pretty good language. It has functional features, even if not as extensive as F#, and has an OO system that is much more well thought out than Java.
Given that C# and Java have copied features from each other throughout the years, and as someone that uses both stacks since their introduction, I wonder what well thought OO features does C# have that Java lacks.
It’s just cleaner and less verbose; eg interfaces can be implemented privately outside of the class namespace, generics are reified, etc... But then there are things like Extension methods, for example, which are incredibly useful.
Generics being reified are a CLR feature available to any .NET language (which Java in the form of J# could take advantage of [0]), don't have anything to do with C# OOP model per se, and were an impediment to properly implement Scala.NET.
This will be fixed on Java side, with value specializations as per Valhala roadmap.
Extension methods are also a way to create lots of spaghetti code, trying to track down where the method is actually implemented.
Swift and Kotlin codebases are good examples on how one can go a bit too far with extension methods.
Java's solution is to offer default interface methods as an approach to traits, which was copied by C# 8.
Regarding interfaces, if you mean explicit interface implementations, besides forcing casts everywhere I don't see what is so well thought out about them.
Generics being reified in the CLR are entirely biased to C# (and vice versa), which is exactly why they impeded scala.net. They were simply designed for each other.
Extensions methods are great, tracking down the methods that you are calling is super easy in an IDE. The ability to abstract over generic type parameters as well as class subtype is very useful.
Default interface implementations have no connection to extension methods, they solve different problems.
Explicit method implementations are great at avoiding name collisions as well as accidental ones. The coercions needed are not hard casts, though C# could do be improved with a soft cast operator.
For all intensive purposes, the CLR evolves with C# in lockstep. Multi language might have been a goal at one time, but isn’t really anymore. No one really uses C++ CLI.
Valhalla won't solve all issues with erased generics in Java. In C# "static T foo;" is a valid declaration that creates a separate "foo" for each specialized class.
> eg interfaces can be implemented privately outside of the class namespace
Not sure what you're referring to here (you can implement interfaces of other packages in Java), but everything else you mentioned is not related to C#/Java's OO system.
