Working in custom application development and consulting I have found, quite often, that clients tend to miss the value of the process.
It seems that success, of the project, all hinges on that initial momentum and it's often hard to get moving.
Usually when a client comes with an idea they feel that they "have it down." However, when you come to them with questions that make them say "well I hadn't thought of that" you get that momentum started. This is, of course, followed by "How much are we talking here?". The client will want to know the cost, right then, right there.
I grew up with a father who is a contractor, building houses, adding additions, etc... I grew up with the idea of 'you can't build it without a plan.' However when it comes to a digital structure, like an application, clients often struggle to see the value of putting together a 'blue-print'. Often the feeling is 'this is a simple idea/project' but more often then not there are many layers below it.
I've come to think of it like a cherry...I know, bare with me... Clients often come with their ideas, they know, or they think they know, exactly what the project needs to be done. This is the cherry's pit. However, almost always, when it comes to the discovery/planning meeting you start to uncover the meat of the project and find out that there is a lot more to it and, in fact, this idea could really be part of a bunch of projects that need to be done.
A friend and colleague of mine has another great post on his point of view in regards to this same subject and he describes it perfectly. His point of view is that the client come to you with a book review and says 'Here's the review...now write the book!'