Really, and this is the programmer side of me talking, you have to taylor-make the controls to the driver/operator. Everybody's brain is wired different, so there isn't any one great answer.
In 2006 (I think) I saw a team that had a mechanum drivetrain that used a single joystick drive with a twist (

pun)... they added a twisting handle to one of the white joysticks to control the spin motion that a mechanum drive is capable of. Cool idea.
As a driver (hopefully I'm not alone on this and someone has a similar experience) some people just 'get' a tank (2 joystick/differential) drive system. Our driver we had through the 2006 season wouldn't take anything else really... he could feel the way the robot drove and found it simplest to do tank drive.
Personally, I agree. Our robot runs most efficiently if we run down our homestretch going forwards, grab the ball, and use the bounce of the ball running into the opposite wall of the field to speed up our acceleration comming back on the other side of the track. This requires me to be able to drive the robot backwards, something that I think would be much more difficult with any other kind of input system I can think of.
-q