At one point within the history of UX design there has been almost hysteria for simplicity, with many of us claiming that simplicity was always good and complexity was always bad. Lately, though, it’s become clear that not everything may have just one button, and pursuit of simplicity by omitting important functionality isn’t good for user experiences. A DEFINITE level of complexity is sweet so long as it adds value through functionality. The optimal level of complexity any given certain product is completely depending on the particulars of the product, so it’s impossible to define a sweet spot for each possible product.
But for signup forms, it’s possible to understand that sweet spot: the appropriate level of complexity is zero. There have to be no complexity in a signup form as it provides no functionality and price that the user actually wants; all of the value is for the web site owner.