Yellow: lack of oxygen, probably.
maybe from kissing too long?

Mr. Jimbles: Benzodiazepine, I'd say.
maybe...when you know that they've been thinking of you?
even when you're not together?
smiling when they see you
prioritizing you above other people (within reason)...like if they're talking to friends and see you, they politely end their conversation and come to you?
i don't know, i don't really "swoon"

Optimistic.: I've always liked it when a woman enjoys things that seem to be stereotypically masculine
i.e., she can talk about sports
also, if she's independent
nothing less attractive than someone who can't live without you constantly by their side, you know?

Cognoscente: I dunno
I like independent smart girls I share interests with
everyone is different
she should just be there for him, I guess

Laser Jock: Hm, food (whether baked goods or a meal) is always nice. Going out of her way to see me.   Maybe planning something small and spontaneous (though that's a bit bigger than flowers). Leaving little notes.
(These would all work pretty well the other direction too, I think.) Hey, even flowers could be nice, though I admit that might depend on the guy. If you're including small gifts too (in the same price range as flowers), it would just depend on what the guy likes. For instance, a girl once gave me a book she thought I'd like.   (Paperback, $8 cover price.)
(I ended up deciding I loved the author in general, so it turned out to be quite an appreciated gift. :) )

Sauron: creative, shared interests, easy and frequent conversation that doesn't get routine.
I dunno, [Waldorf] does crazy creative little things to surprise me sometimes.   She draws a doodle for me or writes a story or makes a comic.

Quandary: maybe this is too nerdy but I would say intelligence does more than anything else.

PD Kirke: I enjoy being surprised by girls.
Not jump-out-from-behind-a-corner surprised, but intellectually surprised?
I did a kind of elaborate homecoming invitation this year, so the girl I asked did an elaborate response, and as part of a code I had to find the third semiprime number with a semiprime aliquot sum.
I mean I was expecting something crazy, but no one can expect that.
I'm much too manly to swoon, but if I did swoon, that would have been a swooning moment.

Daniel (blog commenter): Unexpected attention (surprise visits at work, periodic unnecessary (and brief) phone calls to talk, etc.).
Taking initiative (i.e. periodically being the one to plan and execute dates, invitations to dinner, generally things that aren't done together (which are a must, of course) but are done for me).
These are the two biggies for me.