I commute about 22 km each way year round, rain or shine (or snow). The ride goes from pretty quiet suburb traffic to very busy city traffic close to work.
Most of the year I use clipless pedals (Shimano M-540), a steel frame touring bike, 28 mm tires, and a backpack. I wear a full spandex bike outfit in bright colours. I've got a wind jacket, a gilet, arm and leg warmers, a buff, and some thin gloves for cooler/rainy weather. I always wear a helmet (with some reflective tape that I cut and applied to the back), and usually ride with a backpack (change of clothes/lunch).
During the winter I have an aluminum frame road bike set up similar to my summer one, except that it's got bar end shifters (so I can shift with heavy winter gloves on), fenders, a rear rack, 32 mm tires with a little bit of grip for snow, and studded flat pedals (so I can wear warm winter boots). Winter cycling gear is a mixed bag of whatever works in very cold temperatures. I have some heavy weight tights, several warm sweat wicking fleece sweaters, a bright neon green windshell with lots of reflective stuff on it, a neoprene face mask, an old bike helmet with all the vents taped shut, a touque, ski gloves, wool socks, and insulated hiking boots. Usually I'll use panniers in the winter because I find that my back gets super sweaty and uncomfy with all those layers and a backpack on top.