Poor writing, which is not quite the same as poor editing. I've read books by really good writers that had mistakes that would have been caught by better editing and proofing. I can usually deal with those because they are generally not too numerous, and the overall impression of the writing is good.
What will make me put a book down is a
lack of writing skills. Poor grammar, poor spelling, and do not even get me started on comma usage.
But then, anyone who was around for the "Ugly Truth" series of threads probably already knows
how I feel about bad writing.