Of the three, my guess would be making web-form creation accessible to people who could not do it before. While it's great to save time, that's a percentage improvement; whereas making the impossible possible is of a different order altogether. It's also to get people to adopt if they haven't already invested in a way of doing it that is already working for them (even if it is inefficient etc) - change is hard when you have bigger problems to worry about. But that's just my guess - I'd love to hear from the wufoo team...

But I agree with you about the support, design etc. There are a few companies providing online forms. Technically, it's not that hard to do; and I had a quick play with a competitor, that seemed pretty good. So I guess the "what people want" above just defines the mini "industry" of online forms - if Wufoo's is within an industry, its success must be based on additional factors, like the ones you mention.