I've found that I can cut a book / series a lot of slack if it has well drawn-up characters. Conversely, I've found that I have a really difficult time slogging through something that, on a fundamental level might be a masterpiece, but which on the surface is plagued by uninteresting plot mechanics and shallowly-defined characters.
Case in point, The Lord of the Rings. Fantastic, classic series. Astonishing bit of world building, and an endless treasure trove of symbolism. Unfortunately, I've never been able to get through all three books (not counting The Hobbit) because the characters are cardboard cutouts, and despite everything that happens in the story, it all seems to happen to the characters. Nobody seems to have all that much individual motivation to make it happen themselves. They fall back on the necessity of the matter (i.e. the Ring must be destroyed--well, duh!) and let that guide their every move, as though they exist for no purpose other than to ferry the Ring to Mt. Doom. And even when they do show some initiative (e.g. Frodo leaves the Fellowship; Aragorn, Legolas and Gimli track the Uruk-Hai to attempt to retrieve Merry and Pippin), it's usually the obvious choice, and it never feels like much thought or emotional consideration is put into it. Did I miss the line somewhere where it states that everybody in Middle Earth is descended from Vulcans or something?
I'll throw it a bone. The story is told from third person perspective, which makes it more difficult (though certainly not impossible) to personalize the narrative, and it's meant to be read like a historical account, so the dry tone is perhaps appropriate, if notably dull, IMO. I'm not saying it's a terrible piece of work by any stretch, but I personally have a lot of trouble staying interested. The movies, for all their faults (e.g. the inexplicable plot alterations, the Arwen / Aragorn love scenes...ugh!), nonetheless manage to inject some personality into the characters. I feel like they are involved in what's happening, like they're aware of the consequences of their endeavor, and that makes it easier for me to feel involved as well.