In reading over the reviews it is easy to find people who are quite hostile to what Nielsen has to say and it's easy to understand why--he insults and threatens their very approach to web design. Nielsen's message is really quite simple: web sites should be constructed for the end user, not to demonstrate the skills and ego of the designer. Unless you are designing a site that is intended simply to demonstrate the breadth of your abilities, designing is not about fun--it's about taking information and making it as accessible as possible for your end users. It can be fun, but your own enjoyment shouldn't be the purpose.
In Designing Web Usability Nielsen does an effective job of demonstrating ways in which you can help your users to move through your site efficiently and accurately. Individual sections may seem seem somewhat self-evident, but taken as a whole these sections add up to an impressive amount of information.
Of particular interest are chapters 4: Site Design, and a section in chapter 3 on writing for the web. In the section on site design he discusses issues such as using navigational cues to orient the reader to where they are, where they've been, and where they can go, and different types of organizational schemes. As a technical communicator I found the section on writing for the web particularly relevant. Put simply--most people do not like to read online. You have a second or two to grab their attention and only a bit longer to hold them there if they are intrigued. Relevant information needs to be placed front and center and should be foregrounded through the effective use of headings, bulleted lists, etc. This section does a good job explaining how to do so.
None of this is to say that Nielsen is without fault. I do believe that there are times when his rules should be broken. He seems to assume that conveyance of information is the only purpose of the web and, obviously there are many other reasons people surf. Nevertheless, if your site has the purpose of communicating information of some form, this book should be read and absorbed.