Partly, it's simply the variety. "SF" can be pretty much any genre of fiction, from superheroes to other boys' adventure stories to romances to mysteries to war novels, even Westerns of a sort.
Further, even in the context of a particular genre, the SF element can have a novelty of its own, some exotic new weapon in a war novel, or exotic new tech in a spy ovel, or an exotic jeopardy for star crossed lovers in a romance novel. Of course, SF still trades on nostalgia for the stories of youth.
The rest of it is the basic attitude. SF as a mode is based on the idea that the future will be different from the past. Although anyone who's been alive very long knows this is a fundamental truth of our human condition, there are lots of writers who don't seem to have noticed. (Unfortunately, these include some SF writers, particularly the latter day Trek incarnations.)
Additionally, SF as a mode is based on the idea that the material universe is a part of the human condition. This too is inescapably true, despite the number of writers who would deny it. Actually playing with ideas about the world around us is entertaining, because it's provokes thought. There is very little SF that swamps the reader with heavy duty science, especially that appropriate to a textbook. Generally most complaints about technobabble are complaints about big words.
If you must couch the attraction of SF in negative terms, what it is not, it is not literature predicated on the absurd and reactionary ideas that human nature is eternal; character is destiny; the sole purpose of literature is insight into the psyche, not society and/or nature; society and nature are fundamentally incomprehensible; things just happen; there are other modes of knowledge than science.
As to a definition of SF, it is a nonrealistic mode of fiction that mimics realism by rationalizing the fantastic elements in it as possible with greater knowledge of the laws of nature. The plausibility of the rationalization is dependent on the knowledge available to the writer, his or her powers of invention and, most problematically, the sophistication of the reader. This is why the plausibility of the fictional science is relevant to how successful the SF is, but doesn't define the mode. Nor is it relevant what particular genre, if any, fiction written in SF mode happens to fit.
There is confusion about fantasy as a mode. In the literal sense, all fiction is fantasy, even Zola's La Debacle. Fantasy is a nonrealistic mode of fiction that uses acknowledged impossibilities without rationalization within the fiction. There is no pseudorealism in fantasy, however much it may adhere to the standard beliefs about human nature or society or whatever.
Conservative people who don't really believe that tomorrow will be different will often confuse SF with fantasy. Also, people less prone to reflecting on structure, theme, style will not notice how the pseudorealism of SF distinguishes it from fantasy. People who view all fictions as entertainment that either appeals to wish fulfillment will be indifferent to the difference between fantasy and SF, because that is what most fiction seems to be. This appears to be a reformulation of Sturgeon's Law, which says that ninety percent of everything is crud.
Thanks for the compliment, xortex. 
