From my experiences...

Drivers in (and just south of) San Francisco on 101 and 280 seem to think that the left lane is reserved for those desiring to go 10 miles an hour below the speed limit.

Merging drivers in Michigan seem incapable of controlling their car speed to arrive at the merge point before or after you (as the driver in the right lane) reach it -- rather, they become belligerent when you don't slow down to accomodate them. (I always try to move over if I can, but it's not always possible.)

While not an odd driving custom, I would like to add that I'm consistently impressed with Brazilian drivers. While many are aggressive, they all seem to get along pretty well on the road -- slow pokes move over when someone comes up from behind, multiple cars passing one vehicle almost do so gracefully, drivers make space if you're in the middle of a tight pass, etc.

In contrast, on a recent trip to Malaysia, I was floored by the low quality of the drivers, particularly their habit of slowing down to a crawl in the driving lane before making a left turn (rather than using the shoulder.)