It never ceases to amaze me what a difference "presentation" makes.
Pizza Hut is airing a commercial around here about their "Tuscani" menu. In the commercial, they show people doing the old "Surprise! Your coffee is Folgers Crystals!" trick in a fancy restaurant, except they're serving Pizza Hut food in an "Olive Garden"-style venue.
It clearly shows my point, and that the point applies to anything... books, food, appliances, vehicles, and software, just to name the first few things that pop to mind. You can have the greatest product in the world... it exceeds expectations in every functional way... but any adjective that is instantly applied to the visual presentation (including the environment it's presented in) will be applied to the content.
If it looks like crap, that's what people will think of it.
(Of course, there are two sides to the coin... What really kills me are the times when a really polished application really IS crap... it's UI is very appealing, but not thought out. It crashes at every click. But it looks BEAUTIFUL. And so people love it, at least enough to be sucked into buying it.)
Good engineers don't go for the adage "It's better to look good than to be good." We know far better than that. You can't judge the power of a car by its steering wheel. Granite countertops look great, but they're typically hard to keep sanitary.
When it comes to application user interfaces, engineers tend to make it function great... it gives you the ability to control every nuance of the solution without allowing invalid input... but if it looks kludgy, cheap, complex, or gives hard-to-resolve error messages, you get those adjectives applied to the whole system.
So what I'm talking about, really, is a risk... and it's a significant risk to any project. For that reason, appearance litterally becomes a business risk.
For any non-trivial application, a significant risk is end-user rejection. The application can do exactly what it's designed to do, but if it is not presented well in the UI, the user will typically tend to reject the application sumarily.
That's one thing that I was always happy about with the ISIS project. (I've blogged about our use of XAML and WPF tools in it, before.) The project was solid, AND it presented well. Part of it was that the users loved the interface. Using Windows Presentation Foundation, it was easy to add just enough chrome to impress the customers without adding undo complexity.