One difference I think about is generics. Java and .NET bolted generics on to an existing system. Java used type erasure in such a way that a List<OfThat> is really just a List. Type erasure has a lot of limitations. If I am coding in Java for days I never get into trouble with it because I know how to color in the lines. But do some balls-to-the-walls metaprogramming and then it is annoying that you can't write

   Expression<Integer> add(Expression<Integer> a, Expression<Integer> b);
   Expression<Double> add(Expression<Double> a, Expression<Double> b);

because in the end they both look like

   Expression add(Expression a, Expression b)

we have ways to cope, like unerasing the types by rewriting the names... And now you've got a reason to do balls to the walls metaprogramming! Similarly if I do a lot of C# or Scala or something I will get into the habit of doing things I can't do in Java.

.NET on the other hand did not keep backwards compatibility, so a List is not a List<X> so .NET had a schism where some API functions use generic collections and others use non-generics which was annoying in its own way.

Something like that is how all methods in Java are virtual whereas methods in C# may or not be virtual. All-virtual is probably not the best for performance, but it is simple for understanding. You never have to think "do I make this virtual or not?" or think "is that method virtual or not and does that have consequences for how I use it?"