According to Krug in Chapter 1 "don't make me think" is the first principle of usability, the reasoning is that every tiny question adds to the cognitive workload, which distracts and detracts from the usability of a site. Another point from the first chapter is that usability issues can erode confidence in the site and publishers.
This is kind of interesting when you look at the web site of usability guru Jakob Nielsen useit.com, which is crammed full of usability studies and reports, haphazard navigation. Certainly doesn't practice what he preaches, yet he is an extremely credible (often criticised "web design guru that web designers love to hate") expert in the field.
The aim, says Krug is to make everything self-evident or at least, self-explanatory. The oft-cited reason for this, that visitors will just go elsewhere if they are sufficiently frustrated, is only sometimes true. The more important reason is to make pages effective, so people can more easily attain their goal in visiting the site.
How does a company like Microsoft view the question of usability? www.microsoft.com. A formal process, approx 900 users per month test Microsoft's products either in their usability lab in Redmond, through online surveys and field visits.
Microsoft usability research focuses on how people interact with software. The information we gather is translated directly into product design improvements..
So the next time I curse at the way Microsoft have designed a piece of software I should remember it has undergone intensive usability study and consider that the problem might just be ME! :p