For me a book must not be boring or stupid. There's a whole lot of so-called Science Fiction from the 1970's that falls into both categories, in addition to not being anything I'd call SF. Unfortunately a lot of it was published in the pages of Analog. (IIRC that was also one of the magazine's worst periods for sales.)
The story must be presented in a way that comes across as plausible. If it's being called SF then there has to be _something_ in it that's not your everyday existence, but also not something that could be found or created right now. (Or isn't likely to be found/created.)
One particularly bad example (published in Analog) was about a boy, born without arms or legs, who fantasizes about being the star attraction in a freak show. He gets his sister to make him a costume (which hangs on the wall and he never wears it) and his nurse makes him "rape" her several times. That's certainly not science fiction! It and several other stories that could happen to ordinary people in ordinary times, made me wonder just what the bleep the editors at Analog were smoking.
For Fantasy stories, I'm a fan of "hard" fantasy. In other words, fantasy that's internally self consistent. Nobody does something out of the blue just because it's needed to plug up the plot holes.
I call series like Piers Anthony's "Xanth", Terry Pratchett's "Discworld" and Robert Asprin's "Theives' World" hard fantasy.
As for Asimov's "Foundation" series, I felt the ending was utterly LAME. That's one theme that makes me gag. I won't spoil the ending for those who haven't read the books. I much preferred the original trilogy and missed all the atomo-this and atomic-that and the rest of the period SF stuff. He altered the whole theme of the universe in his continuation, and fell into the 1980's "tie-in" fever that so many SF authors caught where they were taking just about everything they'd ever written and munging them all into a single universe. Asimov blended his Robot stories into his Foundation universe.
Another great author who should've left well enough alone was Arthur C. Clarke. His collaboration with Gentry Lee on sequels to RAMA was pretty good, up until about the last half of the last book, where they launched into another of those plots that are just so, "gahhhhh!" inducing.
Now some praise! William R. Forstchen, history professor and author of one of the best (and longest) series of books that involves alternate history (though not much and only as early background), other worlds, aliens, steampunk, huge and very well written battles involving thousands, valiant heroes, brave enemies (on all sides), traitors and intrigue and lots more. There's nobody who writes a huge 1860's era land battle like Forstchen. Most of the books run to over 400 pages and most of them I've read through in one sitting because they're just so good.
"The Lost Regiment" series begins with "Rally Cry" and 'ends' in book #8 "Men of War". He's since continued it with other books with other characters in the same world.
August 1st, 2nd, 3rd, 2014
"I am a machine. I am a weapon of war. I am a destroyer of life in the service of life, the sword and shield of my human creators." Bolo Invincibilus
, Mark XXIII, Model B (Experimental) 0075-NKE "Nike".