Mike is quite correct about the wind and rain. Be very careful when opening the car doors in windy conditions. I know of at least one case where the wind was so strong that it bent the metal that the car door is mounted to and that type of issue might not be covered by the insurance. Broken windshields and water damage from fording rivers is not covered either, but I suspect that your won't get off Highway 1 (Ring Road), so the river crossings are something you are likely to avoid..
Rain coverings are essential for your gear and if shooting from a tripod, always keep a hand on it. Wind guts seem to come out of nowhere. I had rain covers for all my cameras along on the trip and used it. Sometimes it rains so hard, the rain seems to be "falling" horizontally. In those conditions, just stay in the car... On the other hand, if it is snowing hard, do like we do in the snowy parts of the world and just stay put until the snow stops and the roads are plowed.
I suspect that two of my favorite waterfalls will likely be inaccessible.
Svartifoss in the south in Skaftafell Park. It's off the Ring Road but is about a 45 minute walk in each direction. I don't know about accessibilty in November / December. The hexagonal basalt is just beautiful.
I suspect the road to Aldyjarfoss will be closed and it is in the north part of the island, so I don't think you are likely to make it there. Again, beautiful columnar basalt all around.