Usability

I've just been having a think about usability. When someone talks about usability I automatically think about websites, operating systems, consumer elctronics (iPods/iPads) - basically the IT and associated technolgy domains. However, if you think about it usability has applications in many offline/non-tech domains. For example, building, roads, cities, legal agreements are all areas where there can be enormous benefit with appropriate usability design.
I think the key question you have to ask yourself during design is who are your target user(s) and what does he/she need. Are you working on something that will have a lot of users that have fairly repetitive and typical usage patterns? (example: e-mail system) Alternatively, are you working on something that will be used quite infrequently but will need a lot of fail-safe capabilities? (example: Space Shuttle). How valuable is flexibility? Take Apple's iPod for example, the interface is fairly restrictive and inflexibile. However, for most consumers the iPod user interface has provided the simplicity and ease of use that was sorely missing in competing products. In many domains usability is an after thought. The "build it and they will come" mentality often leads to functional products that are poorly designed in meeting the typical users needs.

Another issue you may have to grapple with is what to do if you have more than one target user group. Nowadays many businesses "segment" their users into groups and follow different design strategies by segment. You see this in retail industry with many companies that have established multi-brand strategies - focused on different target shopper segments. You also see it quite a bit on the internet -- where websites are customised by jurisdiction. For a new venture with limited resources it is sometimes important to tradeoff and chose a design philosophy with the most valuable target user segment in mind.

Also, your target user base may evolve over time and correspondingly require a change in your usability design. Take facebook for example, initally designed for a young tech savvy US-centric user base is now needing to change/evolve its interface to attract a more international and diverse audience. Therfore good usable designs are never ever really finished - thevy evolve over time. Even "Google" which seemingly has had a very static search interface has gone through several design changes. You have to be careful about changing design though because you risk alienating your user base. Especially in cases where the user base has invested in learning a particular interface a radical change in desig can alienate existing user (A la Microsoft Vista).