For AT, hair and height can be an issue.
Shoes? Don't care if you're comfortable in them, not complaining at me about your painful feet, and you can dance in them.
Personal hygiene is always tricky after a long, hot, milonga.
These qualify as part of "the right things to look at." Speaking as someone who dances in a tightly closed embrace, usually in moderately crowded places where the average floorcraft is mediocre:
• Hair? If their hair gets in your face, or yours in theirs, that's an issue, depending on relative height. Long hair can also get trapped under a hand, and "slide your hand under the hair" isn't universally accepted . . . plenty of people find that far too familiar.
• Height? In a partnership where each person takes 50% responsibility for floorcraft, that it isn't important. However, most followers I invite want me to take 100% responsibility,
and they shut their eyes. So, I need a maximal field of vision, and a taller follower becomes a problem. A shorter one is never an issue.
• Shoes? Some shoes "lie" to the wearer about their true footprint. This is mainly an issue with chunky or flared heels (mostly on women's shoes) or welts (mostly on men's shoes). Those are dangerous to the other person when they lead to getting stepped on, especially since they tend to be hard.
• Hygiene? This always matters, but it matters more when someone is pressed against you, because you're going to come away with their sweat, scent, and personal-care products stuck to you, which can make
your hygiene look iffy. You can bring only so many spare articles of clothing to a milonga.
All that matters much less when dancing very open in very generous spaces. The other person's hair, feet, and sweat won't be as near you, and there will be enough distance to see around even a tall follower. In situations like that, I'm less likely to look for the above stuff.
But I rarely dance in situations like that. Two Fridays ago, I was dancing at a place where at the busiest, we each had maybe 60cm×60cm, advanced a half-circuit per tanda (not per tango – per tanda), and people who bumped were getting warnings. So, I danced with zero separation at all times, and thus confined my partners to women shorter than me, hair up, in proper dance shoes, and visibly clean.
My example is an extreme for here, but it's closer to the average than "lots of space and people dancing open." The latter really only happens at outdoor events in summer.