I'm not Bruce, but I used to do some layout work, so I thought I'd throw in my two cents worth.
To me, good layout work requires an eye for readability, functionality, and basic aesthetics; as well as a working knowledge of the tools you're using to do the layout with.
Readability is just that. Can I easily read the final product? For example, I'd give the THB good marks for readability. Simple, readable font with good spacing over a plain background. On the other hand, most WotC D20 products would get lower marks in this area, particularly the D&D books with all the text printed over graphics.
Functionality is a bit more difficult to both explain and to achieve. Basically, does the layout allow the product to do what it was designed to do? In the case of an RPG that means are the rules organized in a way that make sense, and is it easy to find the information that I need? Using the same examples, I'd give the THB slightly lower marks in this area when compared to the WotC books, but both follow a fairly logical structure. Anyone who has been around gaming for a while has surely run across books with poor functionality, where you had to go to four different chapters just to find out all the factors involved in one action. This is mainly the editor's job, but the layout person has input, and often it can be the same person doing both jobs.
Aesthetics is probably the most subjective of the three. Basically, "does it look good?" I also think it's the least important factor in layout as it doesn't matter how pretty it is if you can't use it or even read it. I think the WotC books emphasize aesthetics over readability, and their products sometimes suffer for it.
