Shorts, all year round. (You don't lose heat from your legs, it's the upper body you need to keep warm.)
Long-sleeved top, to cover my arms - plus an extra layer, zip-up ideally, so I can ventilate as required.
Waterproof top if it's going to rain.
Woolly hat (or balaclava if it's really cold).
Gloves.
Radio/MP3 player
It surprises me that a lot of runners don't wear woolly hats in the cold.