Not that I know anything about wind turbines, but I feel that I still should understand your questions...so, maybe you could be clearer?
In any case, I will take a shot at making a few commentaries, maybe they will help in some way.
I am not sure what you are thinking when you wonder if wind turbines have some kind of threshold...I mean, they were designed with a certain functionality in mind, given the wind availability, i.e., wind speeds...so, clearly, the designers were trying to maximize power out of them (torque-speed combination)
As far as weight limit, I presume that you are talking about centrifugal force...yes, blades are optimized taking this into account. Given the maximum speeds that they expect to see, designers make things as big as they can get away with for as long as the stresses at the root of the blade are not larger that some threshold that depends on the material used (modulus of elasticity).
Then, there is fatigue...this happens when a part is subjected to compression and then tension, and the compression and then tension...so, I don't know at what speeds wind turbine rotate and how heavy they are but chances are that a blade is under tension when is at the bottom and in compression when is at the top.
You mention friction, too...I would hope that friction in the bearings is not the main reason of resistance to rotation...the main resistance to rotation comes from energy conversion from mechanical rotation to electricity...you see, up there where the shaft of the wind turbine is, there is an electrical generator being driven by it...the shaft drives a moving (rotating) field that induce currents on the stationary part...this is where electricity comes out of...and if this 'new' currents and their corresponding field did not oppose the one that created them, then you would have a perpetual machine...remember, you cannot create energy, just transform it.
Hope this helps
