I've found 40 degrees (F) to be my breaking point. Anything over this, I am still comfortable in shorts and a t-shirt. Under it I start adding layers. A hooded sweatshirt, shorts, and full-fingered gloves at 35 degrees is sufficient. The key for me is to make sure my feet, hands, and ears are covered. The rest of my body has extra, um "meat" to keep warm. Hands are covered by a pair of pearl izumi full finger cycling gloves I got out of a clearance bin at my LBS, ears are covered with a knit hat, feet are covered by one or two pairs of wool socks (depending on how cold it is).

Last February (temps well below 0 often) I just wore two pairs of tight-fitting sweatpants under my jeans, two pairs of wool socks inside thick soled hiking boot/shoes (my feet get cold easily), a hooded sweatshirt under a heavy winter coat, 3 pair of gloves (full fingered cycling gloves, cotton liner, and heavy winter), covering my ears in the bitter cold winds was a PITA I just wore a heavy ski mask with goggles over my eyes. It was a real challenge to motivate myself to ride since my commute back then as 25 miles one way but I had no other choice (my car was out of commission for the month). Although my commute has changed to a measly 5 miles one way, I don't think I'm going to be commuting much this winter.