Traditionally, science fiction has been a big tent. It's only relatively recently that the general public has begun to cotton on to the notion that there are a lot of aspects covered under that one name; you don't need to go back many decades for the common perception to be of rocketships and rayguns and women in scanty metal bikinis. Today it's far more appropriate for "science fiction" to be considered an overarching genre description that contains beneath it things like alternate history, space opera, planetary romances, and sociological speculation. While ridiculous crap in the mold of something like, say, Gale Allen and the Girl Squadron must still exist, at least it's a hell of a lot thinner on the ground now than seventy years ago.
Science fantasy is one particular subgenre that's been around almost since the beginning. Defining it is a difficult thing: beyond the simple descriptiveness of a subgenre that mixes science fiction and fantasy, I tend to lean more toward those that point to science fantasy works as ones that include specificially supernatural forces in it. Still, it's not a perfect definition: something like, say, Shadowrun would thus be considered science fantasy because of the heavy use of actual magic in the setting, and the Telzey Amberdon stories are science fiction even though, fundamentally, psionics are just a way for sf authors to use magic without calling it magic. Same effect, just different label.
It's what I've been saying - labels are important. To me, science fantasy feels like it describes one side of a coin... so what's on the other?
Allow me to suggest... fantascience. I take no credit for the term; it's far older than me, and is just one of the many terms - "scientifiction" was another one, hence why I call myself a scientifictionist - that were dueling for supremacy before "science fiction" won out as a general genre description. Brave New Words: The Oxford Dictionary of Science Fiction refers to it as an obsolete term, listing no usage of the word more recently than 1947. Plenty of opportunity, I figured for it to be rescued and repurposed.
In the end it's all about appearances, so this is how I would define these two sides of the coin against each other - simplistically, of course, but I'll expand as best I can for now.
Science fantasy is a work of fantasy that looks like science fiction, whereas fantascience is a work of science fiction that looks like fantasy.
How does it work? Let's take a bog-standard image of what's popularly understood, at least, to be fantasy - a wizard casting a fireball spell. In a science fantasy setting, the wizard would not be called a wizard at all, but would manipulate the subquantum echoes of the universe or something else that at least sounds scientific in order to summon a fireball. In fantascience, on the other hand, the wizard might look more or less exactly what you'd expect a traditional wizard to look like, but spellcasting means that nanobots in the wizard's brain are commanding nanobots in the surrounding air to heat up to a white heat and launch themselves at the target. This is more or less exactly how it's done in James Alan Gardner's Trapped - though I may be slightly off, as it's been a few years since I read it.
To put it another way, I figure fantascience as featuring the accomplishment of apparently fantastic feats through ultimately mundane means, whereas science fantasy uses the appearance of the everyday to give an apparently mundane air to the fantastic. The way that the "supernatural" effects are depicted matters a great deal, as well - a rigorously examined and justified and understood set of psi-type abilities, like biotics in Mass Effect, wouldn't be enough in my mind to push something into the science fantasy category; the source of these powers would need to be a lot more mystical or ineffable, like the Force. In fantascience, strange things can be ultimately understood - but in science fantasy, they're probably beyond the ken of all but a few.
The way I see it, there's nothing wrong with subgenres - what's wrong with defining the various flavors of science fiction in such a way that readers can zero in on what they're really interested in?