1a. Small overall dimensions: length should be under 105mm, width should be under 70mm. Light weight is also required.
1b. When held naturally, the center of grip-- the point in the middle of your points of contact with the mouse, that is, for fingertip grip, the point in the middle of where all your fingers touch it-- must coincide with ALL
of the following simultaneously: the center of mass, the center of drag, and the sensor position. You must be able to press the mouse buttons comfortably with CoG aligned with CoM, CoD, and sensor. Note that almost all mice fail this for fingertip grip, leading to inferior physical control. In particular, any mouse where the buttons do not extend at least 2/3rds of the way back is almost certain to fail. This is actually a criterion for all grips, but CoG falls in a very different place with fingertip grip than it does with either palm or claw grip, since you're not touching the back of the mouse.
1c. Must easily be able to push both forward and back with fingertips alone, while holding naturally and able to push the buttons. All mice where the side is curving inwards (moving from back to front) at the point where your thumb contacts the mouse fail this criterion and therefore have inferior y-axis control; this rules out "hourglass" shapes (looking from above), and any oval shape with short buttons.
1d. )_( shape is mandatory for the sides. \_/ fails because, with fingertip grip, all sideways velocity is imparted by the thumb, and there isn't really much holding the mouse down except for fingers that are on buttons, so that shape can lead to accidental lifting or tilting during rapid sideways motion. /_\ fails because you need to be able to lift off a lot. |_| basically has all of the problems of both of those shapes, since the surface of your thumb is round. Only )_( is acceptable.
1e. Some extra buttons would be nice, but conventional side buttons are non-optimal because you can't move your thumb without decreasing control. A side button behind the thumb, which can be pressed by making the thumb flatter against the side of the mouse, can work, but if this option is selected, the profile of the button must be very carefully designed so that it is not pressed by accident during vigorous sideways or diagonal motion. Extra buttons on the top surface, such as at the extreme front left and front right corners, could be a possible option.
1f. All of the usual performance criteria are still in force: a good sensor is needed, good switches, etc.
2. When I use a mouse, I use a Team Scorpion Zealot Jr. I say "when" because my wrists are in pretty crappy shape these days (thanks, Ph.D. thesis!
) and I don't have that much time for gaming anymore, so, most of the time, I use a trackball (CST L-Trac R2).