Jason Fried proposes a method of categorising product features, in order to better prioritise them: obvious (stuff that needs to dominate), easy (stuff that people will do a lot) and possible (things that need to be possible, but that people don't do often). Key quote:
Making something obvious is expensive because it often means you have to a whole bunch of other things less obvious. Obvious dominates and only one thing can truly dominate at a time. It may be worth it to make that one thing completely obvious, but it's still expensive.
Obvious is all about always. The thing(s) people do all the time, the always stuff, should be obvious. The core, the epicenter, the essence of the product should be obvious.