What makes a system 'usable'

The funny thing about a usable system is that everyone has a different definition.

Some people will tell you it has to have a great hierarchical navigation, others that the layout must be clear and concise, others still that the information on the page must be presented in a logical and intuitive way.

The answer really is that its all of these things and more. In terms of your information, you must give people (users are really people) the information they need to decide what to do next, without flooding their senses. In terms of navigation, you must provide logical intuitive (if possible!) navigational structures, but also allow for people who don't respond well to that - also make a good search function and whatever other navigational elements are relevant. In terms of your layout, make sure people see things in the order they need to, can take it all in and decide whether to move swiftly on or go into more depth.

I think it's important to take a holistic approach to user experience design - and I don't mean get the crystals out! While each element can (and should) be assessed on its own, it's the way the parts work together that will determine how well your audience receives your system.