In my time focusing on usability, I’ve found that it all really boils down to one thing. Of course, it is very difficult to cover the complete spectrum with one statement, but here goes:
If it needs instructions or you have to explain it, there’s room for improvement.
This is a solid guiding principle whenever building or assessing anything with regards to usability. Although, I usually tell people that if it needs instructions I haven’t done my job. When I say instructions, I’m talking about manuals and large blocks of text—conservative use of very brief and useful instructional copy is allowed, even strongly advised in most cases.
Think about it this way, how many people do you know that can program a VCR? Even better, how many people do you know that can program a VCR without ever reading the instructions? Given, it is a fairly involved process even if it was done in an ideal state.
Now compare this to TiVo. It is essentially the same functionality, but performed in a much more user friendly manner. Would TiVo be anywhere near as successful if it was as difficult as programming a VCR? Simply put, the length of your instructional text is almost always inversely proportionate to the usability of your product.
Lastly, please don’t take this to the opposite extreme and use meaningless and confusing pictures or icons. You should still provide text and test with your user base, just keep your instructional copy simple and to the point. If you find yourself having to explain how a certain widget or interface works, no matter how nifty it seems, it has room for improvement.