The biggest questions they should answer are: "What are we doing, how will we make money from it, and how much money will we make?"
(It should also answer the question of "What do we do if everything fails?").
I found Guy Kawasaki's ten-part outline ( http://blog.guykawasaki.com/2006/01/the_zen_of_busi.html#axz... ) to be pretty reasonable. I followed it for both of the business plans I wrote (freelancing), which both achieved their purpose, and will follow it again for future business plans I write.
Depending on who your potential angel(s) is/are, you may have to be more "optimistic" than you would for others, as far as your financial projections go.
That said, the real purpose of the business plan is to give potential investors some solid numbers such that they can decide the odds of them getting their investment back, and also to force you to sit down and clarify your plans so that you don't get sidetracked with nonsense.