I know that probably sounds crazy - the other big readers I know always read part of the book, whether it's a few paragraphs, pages or the entire first chapter, before they commit to buying it. I don't. I don't even like when the blurb is inside the front cover instead of on the back where it is supposed to be.
Part of the problem is fonts. If a book has too small a font or it's been printed on cheap paper so the ink bleeds and everything looks like it's supposed to be in bold, I won't buy it. I don't have the patience for it. It's not difficult to have your font be of a reasonable size and on reasonable paper (sure, the latter makes production costs higher, but I don't care about that). If a book looks like it will be hard to read, hard on my eyes, I'm not interested.
And I think I've probably missed out on a lot of great books by dismissing them like that. I'm sure I have. It's one of the key reasons I'm yet to purchase a George RR Martin book - I've never stumbled across one that didn't make my eyes want to bleed. However, once I've bought the book, I'll probably read it anyway. Sure, I'll be disappointed when I open it, I'll sigh and bitch and moan, and the book will have to do a lot more work to win me over, but I'll read it.
I've also read a lot of books that I really didn't like and would never have wasted my money on them if I'd read the first page or two. So it's a tricky habit for me to live with. What about you? Do you dismiss books based on something - some would say trivial - like the font?