You guys have all the right answers

(Except for Mikael.)

So here's the summary of Geekman's and my discussions of the scenarios in this post:

The half-empty wine: I think that it sends a message. That message is: I don't want to waste a new bottle of wine on you/your party. Which seems kind of rude. But although we didn't take a half-full bottle to yesterday's party, we did poll everyone there (physicists) and they did all say that it would have been totally acceptable. On the other hand, as JustMe pointed out, they are clearly all smoking crack instead of doing their experiments.

The nail scissors: can I just say, EWWW. Actually, this one seems to polarise people. I think some people (including me) must classify nail scissors in the same "personal hygiene" category as toothbrushes. No way are you borrowing mine! I would even be happier if I didn't share a pair of nail scissors with Geekman. Other people seem to see them as akin to any generic tool - hammer, pair of normal scissors, etc - and have no problem with the idea of lending them. I think hairbrushes are similar in that people vary hugely with regard to how comfortable they are lending them to others. Interestingly, for some people I've polled, a scenario where the nail is bleeding makes asking to borrow scissors more acceptable (emergency), while for others (including me) this renders it way worse (EW, BLOOD).

Helping yourself to someone else's food: I would only do this if I were totally desperate and there's no chance they would know (e.g. an apple from a full fruit bowl, or a sandwich using things they have lots of). And even then, I feel that it's not really appropriate. But on the other hand (as Geekman also points out), if people were staying with me, I'd hate to think they were hungry and desperate, so I'd be happy for them to help themselves to MY food. So I think this is more borderline for me than the first two scenarios.

Cutting a cake before being invited: in the real-life scenario that triggered our discussion, this was actually part of the previous question. We were staying with acquaintances that Geekman had only just met, and when I gave in to his need to raid their fridge in the night, I still totally drew the line at him cutting himself a piece of cake. But even in a scenario where the cake has been produced at afternoon tea or something, I still feel it's rude to dig in until invited, or until the host has cut it. This is kind of irrational - I admit - but it's an instinct I have all the same.

Male colleagues commenting on a woman's outfit: I wondered if I was being super-sensitive with this one. It bothers me personally that one older male colleague (who used to be HOD, so kind of also my boss) comments on my clothes most days. Either, "Nice skirt," or "You're dressed up today - are you teaching?" or "Jeans and a t-shirt? Must be a non-teaching day," or "New shoes?" I know he doesn't mean anything by it, but it bothers me to think that he keeps track of what I wear, or really notices it at all. And I hate the fact that now when I get dressed in the morning, the thought of what he'll say about any particular item of clothing crosses my mind, and sometimes even influences whether I'll wear it.

The connection with Geekman was that he recently wore to work a jersey/jumper/pullover that he hadn't worn in years and so many people commented on it (just saying, "New jumper?") that he felt uncomfortable wearing it again. So I pointed out that he now had an idea of what it was like to be a woman, except that you get this ALL THE TIME. So we were wondering if I overreact, or if other women feel this way too. Your answers were very interesting and it seemed to me that they patterned with the results I got from real-life people. (Women in my department and some non-university acquaintances hate it; women in physics and chemistry claim not to notice or care). I get the impression that maybe it's in male-dominated departments that the women don't seem to care about this so much. I wonder if that's because anyone who hasn't learned to ignore all that sort of bullshit would have gone postal or dropped out long before they get to grad student/faculty status? In more evenly balanced or female-dominated workplaces, you don't have to develop that sort of immunity, because you can just (mostly) avoid the men who think they get to have an opinion on how you dress or behave.