I have read some mainstream philosophy. I have read much of Anarchy, State, and Utopia, and nothing else by Nozick.

Anarchy, State, and Utopia is the least relevant of Nozick's books to LessWrongers. Try The Nature of Rationality (here is a decent summary of the content) and Invariances instead.

My view of LW is that the aspects of philosophy that I am interested in, that I have not already learned enough about from LW or other sources to satisfy me, are areas where LW pretty clearly beats mainstream philosophy.

I'm not really sure what your saying here, but if you get a higher marginal benefit from reading reading Less Wrong than reading philosophy books, then by all means continue reading Less Wrong. On the other hand, if you have completed the Sequences and read most of the other important discussions here, then you might want to check out what individuals in other intellectual communities have to say about these topics.