Caveat: Everything below this point is subject to change without notice. I'm still working out the exact etiquette that satisfies me, and it probably won't mesh exactly with what satisfies you.
If I read you, it means: - I know you (and like you) in person. and/or - You keep an interesting or illuminating journal and/or comment interestingly on other peoples' journals. and/or - You keep an interesting or illuminating Weblog that isn't Livejournal or Dreamwidth, or you comment on same, and it has prompted me to keep up with what you have to say here as well.
I do not expect to be friended back, though I think it's cool when it happens. The more dialogue, the merrier.
If you friend me, welcome! I've discovered why people are explicit about "It's okay not to friend me back", though - the list can get a bit long, hard to keep up with if absent for more than a day.
If you friend me and want me to know about it, or know why, or friend you back, PLEASE COMMENT SOMEWHERE. DW tends to notify me more reliably than LJ, but in either case I might miss it or not do anything about it due to random other busyness. I sometimes peek at the journals of those who friend me, moreso if they take the time to tell me they did so. :)
Not friending you back is no reflection upon you as a person*. Nor is unfriending you, if that should happen*. I periodically go through and prune a bit, and build up a bit. The process takes time.
I rarely friends-lock posts, though.
__________ * I'm sure it's possible that at some point someone will friend me but not get friended back, or get unfriended, because of something that they have done -- although, Pollyanna-like, I hope such incidents remain as uncommon as they currently are. In any case, should something like this happen, I will make every fair effort to make sure that person knows what the issue is, in private if they allow it.