1) Compact size - certainly no greater than 40mm diameter, 38 is just perfect.
2) Relatively thin.
3) Automatic or manual wind mechanical movement.
4) No display back unless the movement is truly something worth displaying.
5) Hours, minutes and seconds (sub or central) only. Date only if you feel you must. Nothing else.
6) Minimal script on the dial. No, seriously. I don't need to know the water resistance in meters and feet, that it's automatic, or that it has been officially certified to be a superlatively awesome chronometer, officially.
7) Precious metal not required.
8) Round case preferred. Really hard to mess that up. Proportions on tonneau and rectangualr watches can be a bit tricky.
9) On a strap - not a bracelet. Bonus points for including a second strap with purchase (say, buyer can choose from two of black, dark brown, medium brown, dark blue, tan).