A lot of people start with a brand preference first then look at specs from there. I used to run Futaba, that is until the JR XS3 came out. I couldn't wait to get synthesized.

Specs I look at.
TX - FM, only way to go for electric and especially nitro. PCM just isn't as prevelant in surface radios.
RX - Easy for me, the JR RS300 (and soon the RS310) because it's the synth rx that goes with the XS3 radio.
Servo - Although not real popular amongst my fellow racers, I use Hitec servos.
1/10 steering - Hitec HS-5925 digital high speed/good torque steering servo
1/10 throttle - JR Z590, good speed/good torque. Came with the XS3 radio as a steering servo.
1/8 buggy steering - Hitec HS-5945, digital high torque/good speed.
1/8 buggy throttle - JR Z590, same as above.
1/8 monster truck steering - same as buggy
1/8 monster truck throttle - same as buggy

As far as specs go, here's what I follow.
1/10th steering - 70+ oz of torque and as fast as you can afford.
1/10th throttle - 50+ oz of torque unless you're running a lot of aluminum parts and need better brakes, then up the torque to 80+.
1/8 steering - 100+ oz of torque and as fast as you can afford but torque is more important because of the weight of the vehicle.
1/8 throttle - 80+ oz of torque, again because of weight, you don't want to come up short on brakes.