Ditto on post #11. Especially the part about having something warmer to put on if you have to stop.

I have a pretty high tolerance for the cold when riding at a brisk pace down to freezing temps and usually arrive home hot and sweaty.
Had a flat last winter when it was pouring rain, temps in the low 30's, dark, middle of nowhere and did not have any warmer clothing to put on while I fixed it. In the 20 min. it took to fix the flat I was near hypothermia. Warmed up pretty quick once back on the road but the situation could have actually become serious.