In general you want the more difficult control to in your right hand (for righties, less important for lefties because they tend to have a lot more dexterity in their off hand than righties do, comes from a lifetime of living in a right handed world).
So that means Analog sticks, trackballs and spinners go on the right. They require fine control.
8-Way sticks don't require fine control, the buttons are usually more of the fine control item with them, so they go on the left.
Many 4-way games only use one button (and often require more exact movement than 8-way games), thus sticking the 4-way stick into the realm of the "fine control" area, which is why many people (myself included) do 4-way on right, or are ambidextrous with 4-way games.
A lot of right handed people simply cannot handle a trackball in their left hand properly, especially righties who use a trackball on their computer (like me). You also get into problems with right handed people trying to play custom lefty Streetfighter layouts. I can play a 2 button game on a lefty layout with no problems, but I would be useless trying to play a 6 button game on a reverse layout.