His prose is often undistinguished, flat, affectless; there is far too much fat (e.g., his inclusion of enormous amounts of inessential mundane detail -- an overabundance of "product placement" mentions, character actions which contribute nothing to moving the narrative forward nor to building an atmosphere nor, for that matter, truly creating a memorable charcter, etc. -- which goes far beyond that necessary to establish verisimilitude), and so forth.
And, despite the claims of various people for his originality or creativity, I find that an enormous amount of his work is actually very derivative of older and better writers -- not simply hommage, but a rehashing of the materials without actually adding anything particularly notable to the mix.
Again, this is not entirely the case, and when he is good, he can be very good; but these are the faults I've found with his work, which I describe by the term "pedestrianism". Fact is, I would like to like King more -- certainly he's someone I'd have no problem sitting down with over a beer, and I admire his work ethic -- but critically speaking, I've found myself less and less impressed the more I've read, both of his own work and that of others. He just doesn't stand up in comparison to so many others, certainly not when it comes to the verbal magic of good prose....
F.E.: I think my hesitation when it comes to Blackwood is that he can, at times, be quite angular and awkward with his actual prose; sometimes even in his best work, and at these times he, too, lacks that verbal magic. Yet, oddly, he is certainly often one of the best for creating a truly weird atmosphere ("The Willows", "The Wendigo", Incredible Adventures, "A Psychical Investigation", "Episode in a Lodging House", etc.); and at times yes, his prose genuinely sings. But then, he also was so incredibly prolific, that a fair amount of his work -- especially later in his career -- has a strong journalistic tinge to it, so that it, too, is flat and flavorless....
Oh, and I meant to include in my list of modern writers: Caitlin R. Kiernan. She isn't always at the top of her form, but when she is, her prose is quite superb....