Nice topic Mike!
My wife thinks I'm a nut about picks. Frequently I'll get in a "Pick Testing Mood" and I'll have all my good ones laying all over the couch or coffee table testing back and forth between each one. She's patient as she rolls her eyes when I ask "Which one sounds the best" or "How'd that one sound". Eventually she says they all sound good. You don't suppose she's trying to get me to quit asking her how they sound do you? Naaaahhhh, can't be!
"What matters most in a pick" was the original question. To me, definitely tone. But that seems to change. Sometimes a Red bear sounds better than a Blue Chip (but usually it's the other way around). Some days my Dunlop Primetones sound better than both. If you all haven't tried these yet, do so tonight! Order one or go to a store that sells them and "pick" one up. They are the closest in tone and feel to a Blue Chip that I've found yet and they're cheap. I use the 1.4mm teardrop without the grip. I get a better grip without the grips.
Here's how I feel in the same order:
1) Pick stiffness: Totally agree with you. Can't bend or be floppy.
2) Tone: I like slightly bright and slightly dark at the same time and still be able to clearly bring out the bass. I find that any pick between 1.2mm and 1.5mm will accomplish this. I never ever go under 1.0mm. I don't know how Steve Kauffman gets such good tone with his cheap yellow .73 picks but he surely does.
3) Feel through the strings: I like slightly pointy with no bevel (except for a Wegen) It's the only beveled pick I like. My Blue Chips and Red Bears are straight, no bevel.
4) Grippiness of the pick: Super important!! I have rough dry hands and It's hard for me to find a pick that I can grip well. My real turtle pick gives me the best grip of any, but it's also noisy (clicky) and provides bad tone and low volume. Don't know what the big deal is about real turtle, maybe I have a dud. Sure is a nice pick though.
5) Size of the pick: I'm just the opposite Mike. I like smaller picks. My Blue Chips are all STP's (smaller version of the TP).
6) Volume: This would be the only attribute I'd say you may have missed. Very important in jam sessions, not so important at home alone.
I always seem to end up with a Blue Chip in my fingers in the end. Yes they're way overpriced but they bring even more joy to something I (we all) truly enjoy, so I guess they're worth it. Just don't lose one! That possibly brings up another attribute - Price.