This depends a great deal on both which branch of philosophy we're talking about & who is evaluating that particular branch's usefulness.
For example, I find developments in logics, philosophy of science, & general epistemology to be of great interest, and I perceive all three topics to be advancing (listed in order of priority as that goes) as the years go by. I'm sure others feel differently.
It would be hard to get past the fact that, especially between the different branches of philosophy, there is a great deal of "philosophy of language" that is or must be done just to get at what anyone's talking about. But that is, to some extent, true of any field with a technical language.
So I guess all four answers make sense in some sense.
Despite being (IMO) a philosophy blog, many Less Wrongers tend to disparage mainstream philosophy and emphasize the divergence between our beliefs and theirs. But, how different are we really? My intention with this post is to quantify this difference.
The questions I will post as comments to this article are from the 2009 PhilPapers Survey. If you answer "other" on any of the questions, then please reply to that comment in order to elaborate your answer. Later, I'll post another article comparing the answers I obtain from Less Wrongers with those given by the professional philosophers. This should give us some indication about the differences in belief between Less Wrong and mainstream philosophy.
Glossary
analytic-synthetic distinction, A-theory and B-theory, atheism, compatibilism, consequentialism, contextualism, correspondence theory of truth, deontology, egalitarianism, empiricism, Humeanism, libertarianism, mental content externalism, moral realism, moral motivation internalism and externalism, naturalism, nominalism, Newcomb's problem, physicalism, Platonism, rationalism, relativism, scientific realism, trolley problem, theism, virtue ethics
Note
Thanks pragmatist, for attaching short (mostly accurate) descriptions of the philosophical positions under the poll comments.
Post Script
The polls stopped rendering correctly after the migration to LW 2.0, but the raw data can be found in this repo.