That's actually a very good question Roger.

The speed controller is good for a certain number of amps.  Keep under that limit and it wont' fry.  The motor is good for a certain number of watts- volts x amps, and generally also has an amp limit too.  as long as you don't go over for more than a few seconds everything will be copascetic.

The prop is really the ticket.  Unlike a glow engine, an electric motor will try and turn the prop a certain rpm, based on the rpm/volt of the motor and the voltage of the battery.  The more load from the prop, the more current the motor draws.  You can easily fry the motor and speed control if the prop jams in a crash.  The amps go through the roof as the motor instantly draws the max current the batteries can supply, trying to turn the jammed prop.  Fortunately most of the brushless controllers/motors have protections built in to prevent this from happening.

Higher voltage batteries have a lot to recommend them.  Higher voltage means you need fewer amps to get a certain amount of watts(power) and you can use smaller capacity batteries or do less damage to the existing battery.  Lower amps wastes less power in the wiring/batter/motor.  Since the wasted power goes up as a square function of the amps, halving the amps cuts the lost power by 75%.

Well the speed controllers are sold with a maxium ampere rating. At "full" throttle, they are supplyinng full battery voltage to the motor. Depending upon the rpm of the motor (it's "kV" rating) and the propeller you choose (a big  one which limits the rpm to a value which is a lot less than 1/2 times the kV motor rating and the battery voltage), you could put a so many amps through the motor that you burn it out, or the speed controller if it were matched to the motor.