From Futurama: “Insane in the Mainframe”
Roberto: "I'm not crazy. Don't call me crazy! I'm just not user-friendly!"
Your application / software is just like a person. If there is a part that you don't like about it, it can ruin the whole experience. So when you're looking at your application and you think it's fantastic, be careful you may have "over looked" something about it. Shall we say, for example, an over zealous use of animation or gratuitous usage of design patterns? Remember that your overall application user experience (UX) can be poisoned by that one bad feature.
For example, take the Apple iPod, the hardware device is elegant. The wheel is the perfect way to navigate around the device (left or right handed), and the size and heft feels solid and the overall design, clean . But, iTunes spoils the experience. It's a CPU hog, it's hard to transfer videos to the device, and you have to use many third party tools to get it to behave correctly. iTunes does get some things right. Podcasting, buying items is dead easy, the amount of content on the iTunes store and the UI layout.
But remember the next time you’re waiting for those files to encode and you lose a file because something didn’t transfer correctly and you cuss the application that it's UX. Make sure that the sum of your application experiences are not overwhelmed by one or a couple bad ones. And also remember that by focusing on UX, your applications, even if you just fix one part, can start to shine beneath that tarnish of bad UX.