Well, torque is a measurement of the twisting force and the more you have the more suction the fan will generate in denser mediums. For example, if you were opening up a particularily stuck on lid on a jar of pickles, you'd need a lot of torque to open it up. I'm not sure that it really matters for air unless you're attempting to yank air out of small crevasses or a maze of radiator fins. Air is restricted by friction.
I'll give it a shot (I'm a water cooling noob, but this is what I've understood)- Some fans can move lots of air, but when they encounter the additional resistance of radiator fins, their efficacy drops off significantly. For water cooling systems, fans with more fin surface area and steeper fin angles can provide extra push to the air, while not necessarily moving more air when unimpeded- much like the different props used on planes and boats- there are different configurations for different conditions and purposes.
The fans I just ordered are 38mm thick as opposed to the standard 20mm, and as far as I know, are better for radiators.
Hardware Canucks is focused in-depth reviews of the the latest graphics cards, CPUs, storage, smartphones, notebooks and gaming peripherals. Join us as we unbox, review and benchmark the best computer hardware on the market.