The damned colloquialisms.
"F--- me, Freddy" in Dreamcatcher,
"Shoot a Pickle" in The Tommyknockers,
"Doodly-Damn" in Cycle of the Werewolf (thankfully excised from Silver Bullet),
or any other that I can't remember, they're omnipresent and annoying. They grind the story to a screeching halt and never sound natural coming out of anybody. Maybe the problem is simply that I've never in my entire life actually heard anyone say any of these things. Whenever I hear someone in the story say them, they never seem to be in the right context, and even READ like they were afterthoughts. As dialogue in film or television adaptations, they sound even more clunky and forced. It completely kills the immersion and utterly deflates the terror.
Of all the criticisms I can level at King's work, this is the only one I can never forgive and move on from (except maybe the name-branding and pop-culture references).