This is what I'd love to see die the most. All too often logic will be spread out through unmanageable inheritance trees. I've never seen a completely necessary use for inheritance and I've seen tonnes of misuse.

Inheritance is probably misused a lot. Having deeply nested inheritance structures is definitely a bad idea. It is useful in some situations.

Polymorphism on the other hand is a powerful technique to build flexible systems.

So a decent object system is a definite must for you?

What do yoy think of mostly-functional languages that add an OO library?

I haven't worked with a functional language professionally. Closest I came was with python which supports these constructs. I suppose it comes down to trade offs when designing a language and for what purpose you use them.