Late last night/early this morning, I encountered a post by 
Long Story Short, the HAWT requires a lot of force from wind to make the big generator turn and produce electricity. This is primarily because the generator has internal resistances from it's parts which are largely unavoidable in a horizontal configuration. In addition to this, a large blade assembly is needed to catch enough wind to rotate the generator shaft, but again in horizontal configuration the design must also now lift part of the weight of the blades in each rotation, and these are steel due to the structural strength required as a result of their necessary size. The result is a turbine that requires high wind speed to work at all.
In contrast, the Vertical axis design has no requirement to lift the weight of its rotor inside the generator, as the shaft is perpendicular to gravity. The other major difference is the blade design, and these vary, but are made to be far better at harvesting energy from wind than Horizontal designs. Therefore they can be smaller, and since the aren't required to lift their own weight, need not be as strong, and can be made from lighter materials. The result is a turbine which can do it's job even in a light breeze. There are other factors, such as the fossil fuels necessary to mine the resources to manufacture the turbine, and in the case of the generator component this is the same in both designs, but with a turbine that is actually productive it has a realistic potential to offset those carbon emissions. I have seen the blade assemblies made from materials as simple as a blue plastic 45g drum cut in half, recycling as well as not being mined. These are the most significant comparisons I can currently think of. I will edit appropriately or comment if more come to mind.
and the animation of blade motion;
