A lot of Don't Make Me Think concentrates on navigation as a major part of usability. In some ways this dates the book, as navigation is no longer the primary way people find things on sites. It's all about search now.
Don't Make Me Think contains a great analogy about browsing through a store vs. asking for help, and browsing a site vs. using search. One of the points seemed to be that people prefer browsing, both in the store and on a site. They only ask for help/search when they are frustrated by browsing. Thus it is important to make browsing easy and intuitive.
However, how often do you browse a site now? Personally I almost never do, yet I still prefer browsing in a store rather than asking for help. Based on my own experience at eBay, I can say that most of our users are also searchers, not browsers. So what gives?
I think part of the reason people used to prefer browse over search was that search results were poor. Now most sites have much better search engines. If your site is not that big or complex, you can easily take an open source search engine like Lucene and it works great. If your site is more complex, then you may need something more tailored, but you are probably willing to invest in it. But now this argument becomes circular. Why would sites invest in search if people preferred browse, and why would people prefer search if sites did not have good search engines? The answer of course is Google.
People have become used to searching for everything. Unless you are one of those unfortunate souls using IE6, you browser has a search box built right into it. You have to give credit to Google for this. They have become so central to people's web habits that "to google" is a verb. Google has made us expect search. It is the new convention. So every site invests in search.
We expect our search to not only give us good results, but to do more. We expect it to correct our spelling. We expect it to suggest searches for us. We expect the results to be presented in a certain, intuitive way. There are a whole host of conventions around search that have grown out of Google.
Let me give a good example. It's easy to imagine how search is used in eCommerce sites, like the examples given in Don't Make Me Think. However, search goes way beyond such sites. Take a look at Facebook. The other day I wanted to write on my niece's wall on Facebook. I could have browsed through my networks and my friends and found her. Instead I immediately began typing her name in the search box at the top right of the screen. After three letters, I saw her name and clicked on it.
This is a good user experinece. The search was at the top of page, part of the navigation. I started typing in it and immediately got what I needed. I didn't have to put any qualifiers (like 'search in friends', etc.)