SizeThe standard hardcover book, with 9"x6" or 8"x5" pages, about 3/4" or 300 pages thick, generally works well for me. The 8"x5" softcover, about 1/2"-5/8" or 200 pages thick, is great as long as font size is not skimped upon! A book size any smaller than that usually compromises both font size and paper type.
Font and font sizeI prefer a serif typeface similar to Times New Roman. Boring? No, legible! My preferred font size is 12 point or bigger - but not as big as in large print books. I have seen some books printed in a thinner serif typeface, but I find these fonts hard on the eyes.
Absolutely not a sans-serif typeface in a book (and I can't say I like it in a magazine either...) It may look good, but after a few pages, a serif typeface is still preferable for reading. If you must use a sans-serif typeface, at least please do not use the extra narrow version!
Font colorBlack, not a shade of gray (nor a poor print job as sometimes occurs with print-on-demand titles...) Definitely not white on a gray background, or any other unusual color combination!
PaperI like the paper to be thick enough that it feels substantial, that is, a page is easy to grip when turned. This is related to the paper quality, as certain cheaper papers wear out, become smoother, and curl up after many reads. Beware of paper of the type that feels rough to the touch, lacks contrast, and soaks up water like a sponge (not that I would ever spill water on a book ;)
Paper colorWhite, not off-white or grayish.
Hardcover preferencesDust jacket? No thanks, I prefer the visual printed directly on the hardcover, leaving the dust jacket off completely. However, this practice is more common in Sweden than in the US.
Softcover preferencesI am happy as long as the covers are not so glossy that they will warp outward with exposure to humid climates or turn sticky over time, as may happen with certain covers, especially gloss covers.
A popular type of binding in Sweden right now is the so-called "danskt band", "Danish binding" in direct translation, which refers to a softcover with folded flaps of the same material. I could not find a specific term for this in English, other than "softcover with flaps". This is probably my favorite type of binding at the moment as it provides the softcover with a more upscale feel.
More about bindingSince I like my books to last a long time without the paper falling out, the cheapest glue bindings are out. Several of my favorite books have loose pages, and an unfortunate few cracked upon the first read.
PortabilityLast, but not least, portability. If you, like me, like to take a book with you at times, here is my preferred size: 5" to 5 3/4" wide by 8" tall, ideally 1/4" wide, but up to 1/2" wide is ok (yes I measured a few favorites). This format works better for me than the smaller but thicker paperback book.
What are your book design criteria?