What remains valid in Marxist economics?

Tyler Cowen has a go at answering this question and comes up with five ideas. They are a bit focused on early Marx for me. I would suggest:

1. His focus on explaining institutions rather than just assuming them. This is still key to modern political economy.
2. His class-based framework for understanding politics. This is quite a simple schema: the bourgeoisie can ally with the aristocracy or the proletariat. It's still enormously productive. Modern political science, since Olson, has been much less happy with the assumption that classes, which are groups, can automatically solve their collective action problem and come together to defend their interests. But humans seem to be better at this than theory would predict: in particular, they're willing to punish each other to maintain group norms, even at cost to themselves.
3. Ideology. To find out why an idea is popular it is always worth asking: whose interest is it in?