Unboxed values; this primarily affects performance, particularly memory performance, but with a sufficiently smart compiler could be worked around. It's unlikely any very clever JVM optimizations would see wins here though, as the assumption that everything is boxed is idiomatic.

Having a type argument available; as adevine mentions, performing type-level things rather than just instance-level things. A smarter language than Java could handle this with hidden arguments, or passing along a dispatch table for type-specific actions when constructing a generic type. Things get trickier when you're e.g. storing a reference to a static generic method.

In reality, there's a continuum between C++-style generics-as-parameterized-syntax-trees and Java's generics-as-ignorable-type-checker-façade. Going full C++ isn't what you want; the error messages on failure to instantiate are unpleasant. Going full Java leaves you with a few holes. C# is probably a small bit too far in the C++ direction - instantiations can add up, despite .NET reusing instantiations where the type arguments are object references (that is, the CLR will effectively erase types if your type arguments are reference types, but you can't observe this without escaping the type system).