If I downvote with comment, it's usually for a fairly specific problem, and usually one that I expect can be addressed if it's pointed out; some very clear logical problem that I can throw a link at, for example, or an isolated offensive statement. I may also comment if the post is problematic for a complicated reason that the poster can't reasonably be expected to figure out, or if its problems are clearly due to ignorance.
Otherwise it's fairly rare for me to do so; I see downvotes as signaling that I don't want to read similar posts, and replying to such a post is likely to generate more posts I don't want to read. This goes double if I think the poster is actually trolling rather than just exhibiting some bias or patch of ignorance. Basically it's a cost-benefit analysis regarding further conversation; if continuing to reply would generate more heat than light, better to just downvote silently and drive on.
It's uncommon for me to receive retaliatory downvotes when I do comment, though.
A few notes about the site mechanics
A few notes about the community
If English is not your first language, don't let that make you afraid to post or comment. You can get English help on Discussion- or Main-level posts by sending a PM to one of the following users (use the "send message" link on the upper right of their user page). Either put the text of the post in the PM, or just say that you'd like English help and you'll get a response with an email address.
* Normal_Anomaly
* Randaly
* shokwave
* Barry Cotter
A note for theists: you will find the Less Wrong community to be predominantly atheist, though not completely so, and most of us are genuinely respectful of religious people who keep the usual community norms. It's worth saying that we might think religion is off-topic in some places where you think it's on-topic, so be thoughtful about where and how you start explicitly talking about it; some of us are happy to talk about religion, some of us aren't interested. Bear in mind that many of us really, truly have given full consideration to theistic claims and found them to be false, so starting with the most common arguments is pretty likely just to annoy people. Anyhow, it's absolutely OK to mention that you're religious in your welcome post and to invite a discussion there.
A list of some posts that are pretty awesome
I recommend the major sequences to everybody, but I realize how daunting they look at first. So for purposes of immediate gratification, the following posts are particularly interesting/illuminating/provocative and don't require any previous reading:
More suggestions are welcome! Or just check out the top-rated posts from the history of Less Wrong. Most posts at +50 or more are well worth your time.
Welcome to Less Wrong, and we look forward to hearing from you throughout the site.
(Note from orthonormal: MBlume and other contributors wrote the original version of this welcome message, and I've stolen heavily from it.)