Brian Fling just put together a great post about the Immutable Laws of Web Design and Development which listed out a bunch of well known laws about development. Things like Conway's Law: "Any piece of software reflects the organizational structure that produced it.", Occam's Razor: "The explanation requiring the fewest assumptions is most likely to be correct." and my favorite, Hofstadter's Law: "A task always takes longer than you expect, even when you take into account Hofstadter's Law."
I figured I would add my one rule I've developed over my 14 years or so of developing software for a living. It's something that I can't always follow myself personally, but whose veracity I can attest to from personal experience time and time again:
Beattie's First Rule of Application Development: "It has to look better than it works."
This obviously isn't a Law, as it's not particularly specific, but it's a rule that I truly believe in. Users will give the benefit of the doubt to just about anything that looks nicely designed. That is to say, you can have a product that does everything including shining your shoes, but if it looks like crap, users won't ever be happy with it - and any flaws they do find will be magnified substantially. On the other hand, if a user sits down in front of your application, website, widget or what have you and it *looks* nice, it could have a million bugs, and they'll be more satisfied with the application and more accepting of any bugs.
I first noticed this back when I was doing corporate Lotus Notes development and later Visual Basic applications. A prototype that was highly polished but buggy wouldn't bother a client nearly as much as a more complete application that wasn't well laid out, colored, etc. Attention to how an application looks was worth more customer goodwill than its functionality. It could be missing features outright, but if it looked like a professionally done app, then no problem.
Later I've seen this rule pop up on the web again and again, though with some big exeptions - MySpace being an obvious example - but in general, if a site gives a user a "wow" sensation when they arrive, they'll stick around a lot longer than a site that is designed plainly, or not at all.
Now this may sound obvious, right? "Of course sites need to look nice! Duh!" But to many geeks and programmers, it's not particularly obvious at all. The people creating the applications can many times be completely blind to how an application looks. Ever used a program - usually those custom utility apps that come pre-installed with your PC or peripheral - and wonder WTF the developers were thinking? How could they ever develop such a monstrosity!?! The app might do everything you need it to - burn a CD or manage your scanner - but you can't stand it! The first thing you want to do is find a replacement for it, simply because it's somehow "ugly".
The corollary to this rule is, of course, hire a good designer as soon as you can in your development process. I personally am not great at graphic design, so I try to opt-out of the design rush by making things as simple and clean as possible which can work pretty well. But having a real designer add their magic is key, because in short, a good design makes the user like or even love an application before they've even used it.