Eddie Bauer BC Micro Therm Down Parka, Canadian Rockies
Down insulation with a water proof shell in really cold temps. Perfect!
The guys I have climbed with a long time have seldom seen me in a belay jacket. And until recently (the last decade) you wouldn't find one in my pack generally. The first time I heard of a "belay specific" jacket was in Twight's "EXTREME ALPINISM". Oh sure I carried a big down parka on occasion but I can could easily count the times I used one actually climbing on one hand.
The majority of time I used those jackets to sleep in or add extra insulation to my sleeping bag by draping it over the top of me in the tent. Climb in them...not a chance. Too warm. Belay in them? On and off again? In and out of the pack? Going to have to be pretty cold for me to bother...it just takes too much time.
Arcteryx Atom Hoody, Feb., Chamonix, France, 100g of Coreloft
I likely have a picture of every time I have ever donned a insulated jacket to climb in. All but one are recent. And all were in pretty cold (-15C or better). And funny enough none of those jackets are what I would consider even mid weights. More like "hooded light weights." Not having a bigger jacket simply means you have to keep moving to stay warm. You end up carrying less that way. My partners and I have generally been able to do that even in the short days mid winter of Dec. and Jan.
Mt. Snoqualmie, Dec.,WA Cascades, 100g of Primaloft 1
Multiple layers, 100g Primaloft One, MH Compressor Hoody with a
Arcteryx Atom LT mid layer, belaying @ -20C in Jan.
My point to all of this is, you don't need much.
What you do need is simple. Really good insulation first. Primaloft 1 is a good place to start if you like synthetics. Which I do. Makes no sense to me to put on a down jacket when you are soaking wet from sweat and expect the down to stay dry. I save down for the really cold and dry days. The ones where I don't plan on sweating out my base layers. I will work at it to make sure I do not and stay dry.
Don't believe the sales pitch that Primaloft Eco is a great buy. It isn't as efficient an insulator as Primaloft 1 and it a lot less money for the manufacturer. Arcteryx's Coreloft or soem of their own insulations are excellent as are a number of other brand specific insulation's. Again check out the details on the insulation you might be surprised.
Any combo of 60g to 100g insulation should do the trick for usable warmth. Full 100g through out or a body of 100g and a hood and sleeves of 60 is nice as well. Either way check out the insulation combos. Some really smart designs out there.
A decent hood that easily covers your choice in helmets and a matching collar that zips up around your neck to protect you when you are sealed in. Always nice to have a soft chin guard there as well.
Pockets? I like two hand warmers with zips but no fu-fu please just a nylon liner. Outside chest pocket or pockets and internal pockets big enough to dry gloves and ideally a smaller one with a zip closure for the small stuff is always nice.
A simple elastic cuff is what I prefer or a more complicated Velcro closure will work on the wrists.
Outer shell material? Nice if it is durable, breathable and water proof. And a fit that will allow you to keep climbing while the jacket goes over everything you have on. Throw in a two way front zipper to work around the harness.
RAB Generator Alpine jacket punches most all of those tickets as a LWT jacket.