It's not because of the hype. I often rather like things that have been heavily hyped -- I love, to pluck a example entirely at random out of thin air, the new series of Doctor Who -- and certainly wouldn't dismiss a book on those grounds.
It's not because of the weird typography, either. I love weird typography and books that depend on it, from Tristram Shandy to The Stars My Destination. I even wanted Of the City of the Saved... to make more use of it than it did, as I've explained at probably tedious length. And House of Leaves is a massively fun book to flick through, no question. The typographical bizarritude is incredible to behold -- the fonts, the whitespace, the concrete poetry, the pictograms, the photos, the pages of nested footnotes, the upside-down and mirror-printed and non-orthogonal and struckthrough and overlaid text. It's a control-freak author's wet dream, and a designer's screaming nightmare.
Nor is it even the central concept, which is pleasantly chilling -- the house which turns against its owners is far from original, of course, as is the labyrinth which moves its walls and is bigger on the inside than the outside. (The latter, in fact, seemed so familiar that I occasionally found myself wondering if Danielewski was a Doctor Who fan.) The characters depart from their genre precursors in taking a bold and firmly investigative tack, calling in experts in physics and architecture, taking samples for chemical analysis and the like, and generally doing all the things which horror characters usually fail to do because they're fleeing in terror
No -- all this would make a decent enough novella, possibly even a slim paperback novel. And that, my dears, is the problem in a nutshell, or rather in a bloody great mutant nut-husk which has been hollowed out and turned into a boat.
For House of Leaves is simply too bloody long. It is also portentous, pretentious and sententious. The good ideas are ruined by overuse: the footnotes, for instance, work excellently in the chapter where their page-to-page layout becomes a direct parallel to the labyrinth in which the characters are trapped, but elsewhere they're just a joke which very quickly runs its course. The weird typography becomes just as annoying after a while, and the device of embedding the real narrative within a commentary on a film about it which is itself being edited by a borderline psychopath who believes his own life to be altogether more interesting than events, film or commetary takes postmodern alienation to an altogether unacceptable level. Especially since the fictitious editor's life of shagging, pills and maternal insanity is nowhere near as fascinating as he, or indeed Mr Danielewski, appears to believe.
The constant stream of commentary from various critical voices which accompanies the "action" is supposedly, if you look hard enough, satirical. All I can say is, I couldn't find any jokes in it, and I'm someone who finds Frederick Crews' Postmodern Pooh hilarious.
The author biog at Amazon.com reveals that Danielewski is a pupil of Harold Bloom, and I can believe it. Parts of the book read like a desperate plea to enter the Canon of Worthy Western Writings, with repeated references to Milton and Greek myth, interminable Latin tags and, everywhere, the spirit of James Augustine Aloysius Joyce hovering over the waters and doing a good deal of the creative work. An exploration of the subconscious, a retelling of ancient myths in terms of flawed marriages and brotherly rivalry, an examination of language as labyrinth and text as architecture, an oververbalisation of the rumble of humanity's fall -- all House of Leaves needs is some Irish washerwomen and it would be Finnegans Wake, albeit Finnegans Wake as filmed by the makers of The Blair Witch Project. It's basically Finnwank .
If you've the patience to ferret it out, then there's a lot to enjoy in House of Leaves. Unfortunately it's weighted down with accretion upon accretion of utterly self-indulgent pomposity. If I was an uncharitable man , I'd say that Mr Danielewski was so far up himself that it's hardly surprising if he has a terror of being lost in dark places.
 Out of a deep and sincerely felt desire to take the piss, I follow the convention established in the Full Colour edition of House of Leaves, of rendering the word "house" in blue, and all references to
 A word which Purser-Hallard appears to have formulated by analogy with the Doctor Who fan term "fanwank", meaning excessive and annoyingly obtrusive continuity. There's some evidence to suggest that he originally intended entitling this post Finnegans Wank, but apparently he decided against it on grounds of taste.
 Which, on the basis of this post, it's going to be rather difficult to claim that I'm not.
[Edit 3/4/5: To add Bloom links. (Harold, not Leopold.)]