I know that if I drove to the mountains, it would take some damn cold weather to keep me in the hotel. I've ridden in -30 and with the right clothes it isn't all that bad! 40 might keep me in the bar....
-15 for me, fingers freezing is the problem, only way I can keep my hands warm is with big mitts but then I can't work the levers properly to have fun riding.
Rode about 60cm fresh yesterday, -21C was the high. Dressed warm. Didn't notice the cold until the end of the day when the sun went behind the mountain... froze my a$$ off riding the 10 miles out. We played all the way back and no long fast stretches, then figured we'd fly down the trail to get out fast, wrong decision. I would do it again is the snow was worth it. If its just going for a ride with minimal new snow, -15C as a high is where I draw the line.