I also second your point on code reuse. Code reuse/removal of duplication is the main reason behind all programming languages and paradigms. Inheritance is a tool provided by OOP languages. Therefore, to say that it enables code reuse is basically trivial.
http://github.com/raganwald/homoiconic/blob/master/2010/12/o...
As an example, let's assume we're developing a game where cats battle all sorts of other animals for survival. Certainly, the designer has good reason to call the object representing the protagonists in the game Cat. The programmer also has good reasons to call the enemies of the cat Animals, since they will all do the same thing (e.g. attack the cat). However, in this case a Cat is definitely not an Animal. Cat is an overloaded word - it no longer means "feline". Similarly, Animal means something completely different - it's more akin to "enemy". If we could substitute the "natural" ontology of these words with the one our objects define, we would be able to tell that the statement "A Cat is an Animal" is senseless.
I think OOP actually tries to take advantage of the "natural" ontologies we have constructed through experience. This is why this is such a subtle design issue. I'm not quite ready to say it "gets it backwards", but it certainly doesn't mirror the way we naturally look at the world.
I hope I'm mirroring and expanding on what's been said here. I'd love to read your thoughts.
Good design is hard.
The second problem is that re-use is an easy problem to solve in any language. We have delegation, we have proxies, we have so many tools. Why use inheritance, which mixes two different concepts (is-a and was-a a/k/a is-implemented-using)?
Lastly, how is the code more conducive to later changes? In my experience, you now have a problem where changing class A changes the behaviour of every class that extends A. This is a notoriously tricky problem in the real world. In theory, that change ought to change every subclass and inheritance is a win. In practice, the people writing all those subclasses did so making assumptions about the behaviour of A at the time it was written, much as there are billions of lines of windows code that depend on its bugs and break the moment you fix a bug.
Encapsulation is a wonderful thing. I do not see implementation inheritance as having anything whatsoever to do with encapsulation. If anything, I see the opposite.
If I write that B is-a A, if these things are fully encapsulated, shouldn't it be a black box as to how B manages to behave exactly like an A? Maybe there's some shared code, but then again, shouldn't it be possible for B to use some other completely different implementation? That's encapsulation to me. Forcing interface inheritance to be twinned with implementation inheritance breaks encapsulation in my opinion.