For this question, I tend to think of things that are fashionable but outside of comfort zone. And maybe also not "obvious" things like, midriff-baring cropped tops in middle age or short skirts--because these things might vary.
Very pointy toes come to mind, and it's not just physical comfort. I have a poison eye for them as they somehow connote witchy-mean (sorry, fabbers) and also an extreme of not-anatomic shaped clothing--even though of course neither are almond or oval toes and if I really were focused on foot anatomy I'd have to wear Earth shoes or something. So it's that it is more extreme and that it says to me (to me, for me) that I am making a really intentional fashion choice. Hard to explain.
Any really expensive recognizable designer type brand or logo is also outside. But here's where i would have trouble--if it were a super-fit, super quality item (and not just brand hype) and I found it thrifting or e-baying (which I don't do) for a crazy low price, then I'd really be torn, because I'd have to deal with my reality of not having paid any more, and maybe less vs. caring about what others might think. So this can be a problem even with better-quality clothing at sale prices if it is somehow really recognizable, but can be mitigated fewer total items and re-wearing and re-mixing.
High heels, meaning 4" stilletto pumps and sandals come to mind even if I could wear them. They are so not part of any crowd I'm in (except my youngest DD!) and they don't fit the rest of my style either. Now, it would be fun to see if my answer changed if I really could wear them! And I do have HEWI's of comfortable medium-high chunky heels ( 2-2.5", woo-hoo!) and interestingly, just about nobody I'm with ever wears those, either, but I'm okay with that, on occasion.