People sometimes ask me about donations as a "business model".. I always have to tell them: it just doesn't work. If you're goal is to create a business, you simply can't do it with donations.
Except in the very rare case (i think we are talking about way less than 0.5%), people won't donate to support a software project if they don't feel compelled to.
Note: That doesn't mean it's not worth doing! I think we've proved on DC that a donation-based approach can be very successful in terms of supporting a community, bringing people together, helping to offset the time spent by some members, and helping to fund stuff that otherwise wouldn't be affordable. Plus it can be fun once you come to terms with the fact that you have to ask for help from others.
It's just that if you're primary interest is in creating a profitable business, it's a non-starter.
I plug this thing all the times, but you can read more in my article about the first year of DC here