In my opinion, the most important aspect of a rulebook is that it should be easy to use when you play. This is particularly important for the first few times that you have a go with it, especially when not playing solo.
I like the text to be clear (easily understood and reasonably unambiguous), WRG please take note. Diagrams can be useful. A reasonable proportion of white space makes reading easier. Within those limitations though, concise is good because you have a better chance of finding things. An index or contents can be helpful. A5 is a more convenient size than A4, but that is a considerably less important factor than the text being readily readable and understandable when you are in the middle of your first game.
Not fond of the tiny fonts used in the old DBM rulebook, for example. As stated above, the first few times time that attempt to play the rules you will be trying to find things and be able to understand them quickly when you are under a certain amount of pressure because of the other people playing.
Lack of adequate proof reading can make rules difficult to understand. The first version of the Polemos Napoleonic rules had many errata. Can’t talk about later versions because I wouldn’t touch any Polemos rules with a bargepole after my experiences.
Not that impressed with ‘eye candy’ – large colour pictures that don’t necessarily illustrate anything in particular. Although there is nothing wrong with eye candy, it can increase the size and price, and you can always see it on the Internet. I would suggest that colour is only used if it makes things clearer.
Cheap is preferable, bearing in mind that I am likely to buy many more sets of rules than I actually use. However, clarity, good mechanisms and believable results are all more important, in my opinion.