Limit switches are a must for any mechanical device driven by a motor. Hard stops are necessary too, but without a limit switch to tell the RC when the device has reached it's end, the motor will continue to drive the device, even after it's hit the hard stop. Every motor-driven mechanical function I've ever worked on has always had limit switches in every critical place.
We have often used pots to determine the angular location of an arm or device, and I think team 25's arm this past season is the best example of a creative use for pots I've ever seen.
Optical sensors/banner sensors are one of the easiest methods of counting traveled distance accross the field. Paint gray/white/silver wheel hubs with a black matte paint at equal increments, and you've got a decent/cheap/easy counter.
In 2001 we used the gyro/angular accelerometer to self balance on the ramp carrying two goals (in under 30 seconds
).