Please add to list by copy and paste with your additions so we don't have to fall back as it ascends.
Velocity

1) Wet leaves
2) Wet Painted lines
3) Gravel
4)

Yetis. Definitely Yetis.

Those aren't really exclusive to winter.

How about ice and snow?

Flat tires pose a greater hazard, since the clothing requirements for changing a tire roadside are different from the clothing requirements for comfortable winter riding.

New potholes often appear in the winter.

I thought you guys called those "politicians" in PA

Yes potential for all seasons but for winter the list should be expanded.

1) Wet leaves
2) Wet Painted lines
3) Gravel
4) Ice
5) Snow
6) Frostbite

+100.63

7) Fogging, if you are wearing eyewear, like you should
8) Cars and their drivers.
9) Weather: layers

1) Wet leaves
2) Wet Painted lines
3) Gravel
4) Ice
5) Snow
6) Frostbite
7) Fogging, if you are wearing eyewear, like you should
8) Cars and their drivers.
9) Weather: layers7) Yetis
10) Drivers who do not clear windows
11) Sewer, manhole, water main covers (wet metal)
12) Debris hidden under leaves
13) Thinking too much lists to focus on riding
14) Drivers with poor night vision
15) Windburn

Cars skidding on snow or ice

1) Wet leaves
2) Wet Painted lines
3) Gravel
4) Ice
5) Snow
6) Frostbite
7) Fogging, if you are wearing eyewear, like you should
8) Cars and their drivers.
9) Weather: layers7) Yetis
10) Drivers who do not clear windows
11) Sewer, manhole, water main covers (wet metal)
12) Debris hidden under leaves
13) Thinking too much lists to focus on riding
14) Drivers with poor night vision
15) Windburn
16) Cars skidding on snow or ice
17) Drivers who think 4WD makes their car stop faster on ice

18) Pavement immediately before a stoplight where the exhaust from idling cars melts some of the ice, eventually forming a perfect slick - flat ice topped with a thin layer of liquid water.
19) Salt and debris trapped in slush that can rapidly wear moving parts or chemically react with any exposed steel
20) Less daylight

18) Deep rutted ice that is harder than diamonds and never melts
19) Deep very wet snow
20) chain rust
21) drivetrain upkeep
22) salt, salt, and more salt. (see #20 and #21 )
23) short days = riding frequently in darkness
24) crashing, not being able to move, then dying of exposure (maybe that's 2)
25) responding to the inevitable, "You didn't ride today did you?" queries posed by non riders.
26) having to roll with studded tires on dry pavement
27) deep cookie dough snow on top of ice in intersections

28) Other cyclists, ill prepared or inexperienced for winter conditions

29) impatient and distracted drivers who are driving home in the dark when, at other times, would be driving in the light. This frightened me in the suburbs more than nearly anything.

I've found in many areas this gets worse in the evenings and weekends as Christmas nears then begins drops off around Jan 8th or so.

Holiday shoppers are nuts! So what the hell,

30) Holiday Shoppers!

agreed, the darkness makes the impatient and distracted every day driver just that much scarier.

Being on the wrong side of a turning tractor-trailer rig, is year- round but its harder to see you in their Mirrors in the wet and dark season.

30) drivers trying to see out of a postage stamp sized hole in their ice covered windshield while they drive.
31) snow and ice flying off minivans, SUVs and large trucks because their owners just couldn't be bothered to clear the roof.

- Getting avalanched by passing plows
- Snow plowing boxing cyclists into dangerous situations (ie no way to get out the way, or being forced into a bad traffic situation).
- hypothermia

33) the loss of the shoulder or bike lane as the case may be due to plowed snow.
34) Being forced into sketchy deep snow on the edge of the road in heavy traffic and fearing you'll go down and slide right under a car. (see # 33)

Freezing drivetrain

36: Deep breaths of cold air.

37: DOT trucks & their grit/sand/salt spreaders.

38: Any motorist.

39: Ice or snow or both falling from branches, wires, etc.

40: Mist obscuring visibility/icing your bike up.

41: Visions of hot cocoa distracting you.

- Andy

42)Transients with fires, shopping carts, discarded trash and personal items on MUP's
43) Electrical failures on dark streets with abrupt edges