Hey to everyone reading this 4+ years later.
I just wanted to add to what people are talking about forks and "Suspended's" mention of the freehub. I just want to add what I've learned from biking for the past 2 years, year round in Ann Arbor, MI.
To me the biggest thing is salt, but since this was about temps and on the slope I would agree that anything plastic can't be trusted under -10*C unless it was designed for the cold. The seals will contract and leave open gaps, and if it was meant to bend... well it won't. On the other hand, The RockShox Bluto are meant for such conditions, and even ones with lock out can at least prevent wear. Also, those who have mid range forks, and want to use the suspension, Lizzard skins stanchion boots will protect from dirt getting past the open seals.
After heavy (and rather hard) use of my bike I fear that the clutch system has been damaged from the pawls not fully engaging fast enough after slight backpedals. I haven't rebuilt it yet, but I'm pretty sure that this is the case. If you use your bike when it gets -20*C you have to take into account that parts won't move that quickly for you.
Same thing as with the hubs not moving, but this usually happens due to slush. I've had times where ice got onto my rear derailleur and it took a hard bump for it to free up. The font can be just as bad if ice builds up between the post and the cage, but you can knock that with your foot on the fly.
Another worry is that ice can build up on the cassette and prevent the chain from meshing or even shifting to a particular cog. This will usually happen on the one you use the least and all of the sudden need the most.
Over all though, most bikes with the right tires can be used at low temps given that they are cleaned and lubed for the weather. Also, I would love to do more winter biking on snow pack vs. mostly clear (but slushy and salty) roads sometime.