I wanted to know the basic question that a person needs to ask if someone wants his web app done?? It can be UI,platform,crux of the applications and many other things...Please
There are also process questions that may be worth asking intially.
What kind of progress reports would he want to see as you work along on this project? This leads into the whole communication component that is big, IMO.
Would he like to see prototypes and offer feedback at various stages of development in an Agile fashion or is it just a big black box that you deliver when it is done?
Legal requirements, including does the application have a "Terms of Service" or "End-User License Agreement?" as well as what license is this supposed to be built using? Does he want everything built off an open source stack or is it a "just along as it works" scenario?
Scale would also be something as in what are we calling "medium" in terms of its production environment? A medium size webfarm from Amazon, Microsoft or Google would probably contain thousands of machines given the company's size while others may view having more than a handful to be medium sized.
Last but not least, nail down the requirements. This falls into the legal requirements in a sense as you build it for $x and he wants an application that does a,b, and c.