Made me think more explicitly in terms of Bayesian epistemology. I think this would be the biggest boost I got from being a LW member. It helped tons. I'm now significantly better at evaluating evidence and handling uncertainty. In a more general sense than this, the field of probability as a whole and the importance it has in our lives also increased in salience to me.

LW also made the idea of mental models of the world more salient to me. I had it before as well, but not as sharply defined and applicable to everyday situations.

Another nifty benefit was that LessWrong is an important node in a network of very interesting websites, blogs, and thinkers, and it opened up basically a whole new realm full of smart people.

LessWrong is probably the first website I visit daily that can be broadly considered "intellectual" in theme; the rest of them are hobby stuff. It's an important contributor to the fact that I haven't yet let my brain rot.

(This list turned out longer than I expected; only the first item on it was salient to me, but the more I thought about it, the more contributions I could think up.)