After a question from htt://www.anyflower.blogspot.com I've decide to talk about what I think makes good SF. Before anything the story must be well told and good. You could have the most fantastic universe and the most rigourous physics you'll never make a boring book good SF. Then it is important that the universe follow an internal logic it doesn't matter if the science is good or bad if the reader can understand the univers and find glaring inconsistancy he'll enjoy it. For exemple if we take batllefield earth (unfortunatly I can only refere to the movie I haven't read the book) we have in my opinion 2 big inconsistancy that makes the whole movie very bad. First, we are asked to belive that when centuries have past and that cities where abandoned that paper exposed to the element could be found and read by just about anyone (the guy walk in the ruins of the library of congress and find the CONSTITUTION OF THE US intact! even when we see evidance of a fire! and centuries of erosion). If at least there had be a glass case or some indication that it had been protected by something but no it is laying there on the floor. Then there is the whole exploding atmosphere thing. Think about this for a second, the sun is emiting the equivalent of a few thousan nuclear bomb EACH SECOND. you have an habitable planet (by definition close to a sun) with an atmosphere that catastrophically blows up when exposed to radiation.. how in hell could it have prevented itself from blowing up long enought for this alien species to evole and conquer earth. There are other inconsistancy in the movie that just drive the nail deeper in the coffin but I think you get my meaning.
In short SF rest on it ability to tell a good story that follows an internal logic. Asimov robot novel are a good exemple of good SF as are most of is novel. For unscientific novel which follow an good internal consistancy try the first 3 Dune novels (after that porn starts to creep in and the novels get weirder) almost nothing fit the scientific univers but it is easy to belive and doesn't show glaring inconsistency.