People have covered a lot of good stuff regarding what I look for, but specifically:
Organisation. How easy is it to find information within the book? Will it still be easy if I'm trying to do it mid-session, with the players asking questions and making irritatingly high perception checks? Ideally you ought to be able to read the text in full once, comprehend it, then be able to get the most important bits of it from a quick glance back mid-sesh. I recently read the remastered edition of Deep Carbon Observatory, and found it to be my platonic ideal of a well-organised adventure; it's got clear headings, oversized page numbers, and constantly cross-references itself so you're never in doubt as to where to turn for what you're looking for.
Theme and aesthetics. A pure subjective judgement, of course. My tastes are pretty broad, but if it's standard pseudo-medieval fantasy I'm going to need something else to sweeten the deal, like a less common environment (like the tropical jungle from Tomb of Annihilation), or an incursion from/journey to something stranger and more otherwordly (like the weird dungeons in Call of the Netherdeep, or the bulk of the feywild shenanigans in Wild Beyond the Witchlight), or mash aspects of another genre in there (the Indiana Jones-style treasure hunting in, again, Tomb of Annihilation, or the deranged gothic mood of Castle Amber).
The player perspective. Is this adventure actually about the player characters? How easy is it to work in backstories and relationships with the NPCs and histories already in place? I've lost track of the amount of adventures I've read that are perfectly good stories, but consider the PCs as background extras who just happened to have stumbled in; or else they'll have fascinating backstory that the players have absolutely no way of finding out.
