You may notice the tripartite slogan that's part of the logo of this site: Useful, Usable, Compelling. Those three concepts are the pillars of the framework I use to design or analyze a system. In short, what I mean by these terms is:
Useful
Does the thing do what it's supposed to do and only what it's supposed to do? In other words, I'm using your site/app/thing to solve an issue or achieve some goal - does your thing do what it needs to to help me? In order to be useful, the system or tool needs to perform in accordance with my expectations. Does it avoid impertinent or duplicate functionality? A door needs to open and close, not cook a burrito.
Usable
How easy and understandable is it for me to accomplish my goal using your thing? I purposefully avoid intuitive here because intuition should not be a factor. When designing, there should be no assumptions that the user will just "get it." It should be easy to interpret which actions to take in order to make the system do what I expect it to do.
Compelling
How satisfying is the experience of interacting with your system and, more importantly, how does it affect me after the fact? Gilligan's Island and Lost share a basic premise, but only one of them continues to engage you after you're done watching an episode. How well does your system support your brand standards? Does it leave the user feeling the things you want them to?
Obviously, this is a very high-level look at these dimensions of User Experience, but I think they are sufficient to measure how effective you tool is in enabling and engaging your users. I will attempt, in coming posts, to elaborate on each dimension as well as supply tools and tactics to achieve success in each.