1. You're pretty opted-out of mainstream culture- how do you find that affects your interactions with folks who aren't?
I have fewer of them.
This is a really good question, but I don't have much of an answer because I don't have a similar opted-in version of myself to compare -- I've not ever been substantially more or less immersed in the mainstream, so I don't have a temporal comparison, and I don't have any particularly mainstream-invested friends to compare with either. I'm also not always sure where the mainstream is -- I've always had comfortable alternative streams, with nice temperate water and oxygen over my gills, so I haven't been flapping madly towards the mainstream the way I might if I'd been entirely stream-deprived.
Oh, yes, there's one -- I use fewer metaphors with normal people, because I expect to lose them in the middle somewhere. Actually, I expect to lose the weird people too, but they're usually willing to take a minute and retrace the conversational path with more attention, or at least ask questions. Weird people think being confused is fun and challenging, rather than distressing and hostile. (Well, the ones that are likely to get on well with me do.)
Anyway, as I haven't much needed the mainstream, I don't have the kind of practice keeping track of it that folks who are either invested in it or invested in avoiding it do. Trying to blend in, for me, starts with a research phase.
Perhaps paradoxically, I'm not all that obviously opted-out. People don't look at me from across the room and go "Holy shit! It's a weirdo!" You have to get to know me a bit before I start looking funny (assuming that you met me in a fairly mainstream context to start with). Most of my daily interactions (with coworkers, grocery store clerks, other people on the train, etc.) don't involve the ways in which I'm not mainstream, so those differences can't have a direct effect on the interaction. More subtle effects could exist, but then we're back to not having a good comparison. The increasing fragmentation of culture helps, too; people don't really expect everyone to have watched the same television show or sporting event or what-have-you any more, so not being able to join in water-cooler conversation of that type is much less of a handicap than it was 15 years ago. (Also there doesn't seem to be as much water-cooler conversation of that type. Around my workplace, people talk about food and vacations a lot.)
But overall, I don't spend much time in mainstream contexts, so I don't have many interactions with mainstream people, and that's probably the biggest effect.
2. What do you think of Dreamwidth? I haven't even looked at it much, and wonder if I should be considering switching.
From a user perspective, it looks almost exactly like LiveJournal, with a little overlay of hippie open-source hand-made style. I went and got a username over there because it looked like several people I keep track of were planning to switch, but in practice nearly all of them are posting both places, so I don't actually go look very often.
I think it's probably a little nicer as a system -- I don't think it has any ads, and separating you-can-read-my-filtered-posts from I-get-your-posts-on-my-reading-page is useful -- but I've got more people over here, and people are the point. If a large contingent of interesting people flee entirely to Dreamwidth and I need to follow them, I will probably start doing the cross-posting thing myself rather than entirely switching.
3. What is is symbolic of the holidays for you?
For Secular Christmas, which is my particular winter holiday, I require some sort of gesture in the direction of a decorated tree. A pine tree with lights and decorations, a wholly-un-pine-like houseplant with origami birds in the branches, or a string of lights tacked to the wall in a sort of jagged triangular shape are all acceptable gestures I have been involved in.
If one is making a particularly desperate gesture at tree-ness, a gesture at stockings over a fireplace is good to add. The first year my sibling and I weren't in a house with a fireplace, we hung the stockings on a closet rod and created a fire beneath by putting a strand of Christmas lights on the floor, covering the bottom foot or so of the closet opening with iridescent cellophane, and cutting flame shapes out of posterboard and attaching them to a track powered by Capselas so they went around in circles between the lights and the cellophane, creating a sort of flickering effect. More typically, I've hung a representative stocking on a cabinet knob.
The Ur-Christmas in my head is being curled up on a nice comfy couch in my pajamas with a blanket, a nice cup of warm beverage, a Christmas tree softly glowing off in a corner, a fire in the fireplace, and a few family members.
4. What is a cool book haven't I read?
Blindsight. If you don't want to follow the link, we have a nice bound paperback I picked up from the book exchange at work to attempt to lure Andres with.
5. How did you discover fanfic?
You know, I don't remember. I know I started with X-Files fanfic exclusively, so probably I either found some via the old Yahoo directory system under X-Files (I think that was before your time. Once, long ago, these guys thought they could write an index for the Internet. Then they learned better.) or one of the folks who introduced me to the show recommended some at some point. But I think I'd remember a personal introduction, so I'm going to go with found via searching for show information on the 'net.
 That I know of. If you've been trying real hard to be normal, e-mail! Let me know what it's like! (Don't take my not noticing as an affront -- I'm not too clear on what normal is, so my failure to notice you've achieved it does not reflect negatively on your achievement at all.)