wearing a kippah?
I have two reasons. The first, which I offered in those few moments before the service began, is that tzitzit (the reason for the tallit) are a biblical commandment but a kippah is purely minhag. There is no biblical (egalitarian) requirement to cover one's head. (Not talking scarves here, just kippot.) Many people today seem to assume that the kippah is the stronger requirement -- that is, many people wear a kippah but not a tallit during morning services -- but it's actually the other way around.
The man didn't seem to approve of this explanation. I said I honestly don't know now if I'm violating minhag ha-makom (the custom of the place -- that is, this congregation) and no one told me, but I will find out. It's certainly not my intention to violate local custom; it's just hard to tell, as I'm the only woman I've seen lead services. (Most men there wear kippot. I've seen bare male heads on the bimah, but the regular service-leaders wear them.) I will ask the rabbi (who was not there this morning) if he has an opinion; if the mara d'atra, the ultimate authority in his congregation, wants me to wear a kippah, I certainly won't argue.
(Mind, I would have just put one on this morning to make the man feel better, but I was pretty dubious that it would stay on my head without a clip, which I didn't have on hand. I didn't want to create a spectacle of losing and retrieving the kippah, possibly multiple times, on the bimah.)
I said I had two reasons. (Omphaloskepsis alert.) The other is that it feels strange to me to put on a kippah for services but not wear it all day. A tallit is different; it's a time-bound commandment. You don't wear a tallit (gadol) all day. (This may have been different in talmudic times, but I'm talking about now.) A kippah, on the other hand, is not time-bound; to the extent that it's a requirement, it applies all the time (modulo sleeping, bathing, etc). We've all (presumably) seen men wearing kippot at work, on the street, in the grocery store -- they're following that interpretation. It seems to me (and I'm open to alternatives here) that if I'm going to put the kippah on, I shouldn't remove it after morning services are over. But, for better or worse, a woman wearing a kippah outside of the synagogue will be interpreted as making a political statement, in addition to it weirding out my coworkers and friends. If I thought it was a commandment that would be ok, but for a custom? Do I want to go there? (Postponed: a discussion of scarves/hats, which are a different issue in a few ways.)
An alternative view is "hey, it's just a custom -- adapt it as needed". That would make it ok to wear it in the synagogue but not on the street, without a feeling of violating any commandments or perhaps even custom. Lots of people do this, though I don't know how much they've thought about it. Maybe it's fine, but to me the symbolism feels a little weird. (I am not judging anyone else here, just to be clear -- just looking for models and trying to work out my own thoughts.)
Why does one wear a kippah? Various reasons, but the theological ones all boil down to acknowledging one's relationship to (or with) God. Do we only have a relationship with God inside the synagogue? No! To me, taking it off at the end of services feels like I'm saying "ok God, we're done until next time". But God isn't a scheduled activity; God just is.
As I said, lots of people put on a kippah at the beginning of services and take it off at the end. I don't think most people are as introspective as I am, so I'm guessing many haven't thought about why they do this or what it means. But if I've got readers who have thought about this (or are doing so now), I'd love to hear your thoughts.

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That said, it's different for women because the cultural connotations apply only to men. A woman wearing a kippah (outside of shul) is a statement all by itself, and short-circuits the usual connotations.
As to
(*) Commitment to the communities of which I am a part is part of my commitment to G-d; this is a fundamental of Judaism, based on the fact that all people are b'tzelem Eloqim. In Orthodoxy this is often taken to apply mostly to Jewish communities, but even so it is recognized that if you are part of a larger community you have commitments to it. (This is why the more stringently Orthodox minimize their participation in non-Jewish communities; it sets up conflicts.) In Reform, one must face these conflicts; as such, I may be forced to loosen my hold on personal commitments in order to satisfy community commitments.
It should be noted here that, on the level of Torah obligation, eating n'veilah or t'reifah is a relatively minor infraction; in Orthodox communities it has much greater significance because forced kashrut violations have historically been a large part of anti-Jewish campaigns. In a case where I am not being forced or encouraged to disavow Judaism, this does not apply and I do not see a need to rub by kashrut in other people's faces.
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A woman wearing a kippah (outside of shul) is a statement all by itself, and short-circuits the usual connotations.
Right. And a woman wearing a scarf, snood, or wig makes a different statement, which also doesn't work for me. I've got nothing against scarves etc in the abstract, but it might seem pretentious to adopt the patterns of a community that I'm not a member of.
The issue of removing the kippah when, e.g., eating at a non-kosher restaurant is interesting. On the one hand, by wearing the kippah you're an ambassador of sorts, so even setting aside any personal embarrassment, it's not good to give the impression that eating here is fine for Jews. On the other hand, if you can find ways to eat at that restaurant (the notorious fruit salad), maybe you should wear the kippah to tell others (particularly other Jews) that it's possible to be a serious Jew while living in the broader community. Two opposite arguments, both based on the kippah making you a spokesman of sorts. (I know rabbis who follow both of these approaches.)
In Reform, one must face these conflicts; as such, I may be forced to loosen my hold on personal commitments in order to satisfy community commitments.
You're Reform? Or just at a Reform congregation? I'd been wondering where you ended up. (If this is too nosy, of course, just ignore me.)
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On the other hand, it is used as a barometer mitzvah (kashrut generally is), and as such it is assumed that one who keeps that keeps other things and vice versa. I'm not sure it should be used as a barometer, but it is a) fairly public (unlike taharat hamishpacha for example) b) applies to everyone (unlike say, agricultural commandments) and c) affects others (can I eat her food?).