For me a large part of it is how ruthless the domme can be in terms of the breathplay aspect of it. Princess Poison's description of instilling panic in her subs is exactly the kind of thing I love! But like you, I also enjoy the olfactory aspect of it greatly (As my username would probably imply lol).

But also another large part of it is how into it the domme seems to be. For example i've sessioned with many dommes who from my perspective seem very into corporal play. They seem like they're enjoying actually dishing out the pain, and some even lose track of time theirselves during the activity!
But with facesitting it's rare to see a domme who is actually into it (And unfortunately for me none of the ones i've sessioned with who i've had very positive facesitting experiences with are actually based out of NY ), most seem to treat it as a chore and are quick to rush onto other interests I mention.

So I guess the most important factor to me on "What makes a great facesitter" is how into it a domme can seem!