I buy a lot of cookbooks, mostly used. I only buy new ones as a special treat for myself, if they are an absolute must-read for me, like Nigel Slater's new Cook's Book, a new Ina Garten, Canal House. I avoid anything that seems too cute or trendy, and any writer/cook who seems to know less than I do. I am wary of second books after an unknown writer/cook has had a smashingly successful first book, as I always think they'll have poured all their best into the first book, when they were unknown, and now they are just looking for more sales. There are, of course, exceptions to every bit of this.
I prefer metric measurements, and will often order a book directly from England to be sure to get the British edition rather than the "cups" measurements.
Most important is really good writing that feels personal, the writer's true voice and feelings, which is what I love about Nigel Slater, Ella Risbridger, Laurie Colwin, Robert Farrar Capon. I am also apt to buy writers' second, third, and subsequent books if I find, from the first book, that their palate matches mine (James Beard, Rosalea Murphy, Maida Heatter, James Villas, Lee Bailey, Rima Collin).