What makes a tubeless rim?

I am using rims (Kotavelo, Australian brand) that were not specific to tubeless, but manufacturer said they would work, and indeed no problems over the many months since I first installed tubeless. So I reckon it is a case of suck it & see. If you Google the topic, industry is spending considerable time on evolving standards, but we all know how standards aint standards in bike industry. And in my experience, Schwalbe tubed and tubeless tires are equally difficult /easy to install and remove from the same rim (leaving aside the topic of seating the bead for tubeless).

Lack of spokeholes could be one of those things, but mine has holes, so needs tape.  Makes it far easier to build a wheel, do most people use a bit of tape anyway?

A deeper well in the middle?  So as to allow easier mounting with the necessarily tighter beads?

Just speculating.

I believe that it's mainly the hook to hold the tyre bead that a bit different. I can't say that I'd be able to tell by looking though.

I've got some old UST rims on my MTB that have a smooth rim bed and don't really need rim tape to be airtight, but I think it must add some weight to the wheels which is probably why road tubeless relies on the rim tape instead.