1. Comparisons to Tolkien
Every fantasy writer is compared to Tolkien in some way - or at least contrasted. Great as he was, fantasy has moved on, and a comparison to Tolkien isn't just cliched - but a little self-defeating: if you're looking for original worldbuilding, a comparison to the trend setter of fantasy probably isn't a good idea. Comparisons, on the other hand, don't have to be bad. If I ever see a book with, say, this:
"As upbeat and sympathetic as Thomas Covenant! At least half the laughs of Hardy! Less Cthulhu than Lovecraft!"
On the back, I'll read it. For sure.
What do I need to say? If a book isn't making me want to turn the next page, there's probably something wrong. Calling a book a page-turner isn't bad... It just gives a hint of being damned by faint praise.
3. Mentioning a Prophecy
This might just be me. (Well, I'm fairly sure it is me). Prophecies have been done well, but for me, if the book starts, ends, and is described by a prophecy, it's not a selling point. Starting a blurb with a prophecy gives me that impression!
4. Characterisation by Cliche
It's good that a blurb tries to get across a sense of character: after all, that's why we're reading the book. When limited space means that side characters are characterised solely by one/two-word stereotypes and cliches, it's my time to get wary. Especially 'hot-tempered'. No, I don't know why.
5. 'Book One of Seventeen'
There are authors who can pull this off. However, when a blurb starts by introducing the book as a rather lengthy prologue... A book should be a story on its own merits first, rather than using the sheer numerical scale of volumes to impress the reader with the sense of an epic. To give a good example, Brandon Sanderson's The Way of Kings might in turn be an introduction, but its blurb sets us up with characters and situation rather than scale.
Any pet peeves you've got? Comment below and tell me.