What determines the continous ratings of electric motors?
Does thermal dissipation of joule heating limit the max continuous torque?


Yes. Auxiliary cooling or enhanced self cooling can extend the max continuous torque, but that is generally only economically practical with large motors. Th


  Do mechanical limitations (max bearing speed due to friction, mechanical stress, etc.) limit the max continuous speed?


Yes. Bearing friction is usually the limiting factor, but better bearings can extend the maximum speed to the point that mechanical stress can become a factor. Aerodynamic drag (windage) can also become a factor. The size of any sell cooling fan or rotor fins could get to be a major consideration.

There is also a minimum continuous speed / torque limitation imposed by the self cooling limits.


  Are there any "rules of thumb" for predicting the max continuous torque and speed of electric motors?


I have seen some "rule of thumb" on this site that I believe was stated for very small DC motors. I believe it was stated as a percentage of the maximum calculated joule heating.