I'm pretty much in agreement with most cold weather runners -10 is when it really starts to get annoying to run given how much you have to wear. Here's my running gear based approx on windchill temp
25-30 halftights, thick long sleeve, hat, gloves (i typically just wear wool socks on my hands)
15-25 same as above but full length tights.
10-15 add thin undershirt or rain jacket that can be removed and tied around waist
-10-10 trade balaclava for hat, add windbreaker pants.
Somewhere in there, I'll trade normal wool socks for thick wool socks.