My two cents. Background: long-term Leica Trinovid BA 8x42 user, unable to find a modern binocular that really convinces me. Swarovski EL 8.5x42, perhaps.
1) CRITICAL
a) Ease-of-view (Einblickverhalten): No blackouts/beaning, perfect collimation, and undoubtedly other factors which impact on this sensation in complex and user-specific ways. I must immediately rule out many modern binoculars because they give me blackouts (presumably because eyecups too short for my face shape). I also suspect (?) that collimation micro-inaccuracies may often reduce ease-of-view, i.e. cause eye-strain, even in some prime binoculars.
b) Build quality/ruggedness: They must be built to survive field use and to last decades, without frequent problems requiring months of servicing. If an eyecup breaks, for example, the binoculars may be rendered useless in the middle of a one-month trip.
c) Good optical quality parameters, of course: but I really don't care about slight CA, or 90% vs 92% light transmission, etcetera (see below).
2) NICE-TO-HAVES
d) Reasonably low weight: though if this cuts corners on build ruggedness, I'll take heavy.
e) Close focus: It's not often genuinely necessary for birding, but close focus of 2m or less makes binoculars very enjoyable to use. However, I do wonder whether close close-focus may possibly (?) have negative impacts on ease-of-view under normal distance use.
f) Focus feel and characteristics: Often swings-and-roundabouts, in my experience… I like fast focusing, but in some cases this is associated with shallow depth-of-field, which is not so good. Etcetera.
g) Ergonomics: Very important, of course, but I don't consider it very relevant, because in my personal experience there are few binoculars which have significantly poor ergonomics… it's not a common problem for me.
3) NOT VERY IMPORTANT, AS LONG AS NO EXTREME ISSUE
h) CA, FOV, edge resolution, light transmission, colour tint, etc etc. However, one optical feature which may sway me is contrast: a somewhat exaggerated contrast can assist detail resolution. Also, I may sometimes find glare issues significantly irritating.
4) THINGS I WISH L, S AND Z WOULD PAY MORE ATTENTION TO
i) Sufficient eyecup length, if necessary offering a long eyecup option. (I have massive Leica brand loyalty, but I can't consider their current binoculars simply because eyecups too short. That's marketing madness, surely? Or perhaps not: perhaps I'm a rare and not-very-relevant customer type, hey it's their business not mine!) (I also wonder whether eyecup diameter relevant here: wider eyecups will hit your eye sockets sooner.)
j) Eyecup ruggedness: They need to be pretty much unbreakable, and the extension mechanism needs to be solid.
k) Loose-fitting ocular rainguards. Instead of (or as well as) tight-fitting ocular caps, birdwatchers need a loose-fitting long-oval rainguard / dustguard which falls back immediately and freely when you bring the binoculars to your eyes. I mean, I don't think it's just me! Not rocket science.