This is almost exactly what I was about to post. 
I read (and re-read) and enjoyed his short story collection Dreams Underfoot when I was about 13, but when I got around to reading some of his other short story collections I was pretty disappointed in them. I never bothered with any of his novels because they didn’t look very good. Then I re-read Dreams Underfoot again and was pretty disappointed with that.
A friend of mine in college who was underwhelmed by de Lint pointed out that most of his significant female characters are really just the same character, and speculated that he couldn’t stand to write for long about anyone but his fantasy woman. I suppose they might all be based on his wife, which would be kind of sweet, but still it got old.
This same friend also shared my experience with Pamela Dean’s Tam Lin. It had been enthusiastically recommended to us by other friends…but we were not impressed. It turned out later that my sister had the same experience, and I suggested we should form a club.
I like the idea of a 20th century retelling of the ballad “Tam Lin”, and the book started off doing a pretty good job of introducing the fantasy elements into the real world setting…before turning into an excruciatingly dull account of the mundane day-to-day life of a college student. There’s no plot in this book except for the “Tam Lin” storyline, but it went for IIRC a hundred pages or more without developing this at all. Instead the heroine did things like meet with her adviser and plan her course schedule. She also meets Tom Lane (who by his name is obviously a variant on Tam Lin) early in the book but doesn’t get together with him until a lengthy romance with another character who will prove to be IIRC totally unimportant later. I kept thinking “I know you’re going to end up with Tom, his name is the title of this book, so just get on with it already.”
Years later I happened to see Dean’s Juniper, Gentian, and Rosemary at the library and decided to give her another chance, hoping she’d been able to cut down on the boring stuff a bit. Turned out it was even worse. She sets up an intriguing situation early on, then goes almost the entire book – hundreds of pages – without developing this at all until the last chapter or so. It was like she’d written a short story and then padded it into a novel by giving what seemed like a minute-by-minute account of the teen heroine’s homework, what she had to eat, etc.
I gave The Dresden Files what I feel was a fair chance – I read the first 4-5 books – but the things about them I found irritating (e.g. the main character, and how pleased the author seemed with him) did not improve.
I really expected to like Jasper Fforde’s Thursday Next books, but I felt about them the way I think the people here who don’t like the Discworld books (which I love) must feel about them. The first couple had enough promise that I kept going in hopes they’d get better, but a chapter or so into the fourth book I thought “There are a lot of things I could be reading that I wouldn’t have to force myself through. Things that are actually funny, and don’t just have characters with stupid puns (Jack Schitt) or out-of-context common phrases (Harris Tweed) for names.”