It’s really anything that doesn’t match the team’s (or individual interviewer’s) notion of what a good developer looks like. So it could be how you dress, how you talk, even if you are white and male and not over some age threshold.

And I suspect much of that is not conscious, which of course makes it difficult to talk about and address.

I got the job I have now while, for lack of a better term, being "visibly trans", the only person of my race on the team, and also well into my 30s. I don't think it'd be possible for me to look more different than I do from my teammates (especially how the team looked when I started - it's grown and become somewhat more diverse since)

That said, I do worry about how my presentation will be perceived elsewhere, it comes time to move on.