Escapism isn't a very good word, actually, for the positive psychological qualities its defenders want to defend; it's less a question of breaking one's bars and running away (running whither, we might ask?); it's more about keeping alive the facility for imaginative play, that faculty that only a fool would deny is core to any healthy psychological makeup. Kids are good at play, and have an unexamined wisdom about it; adults, sometimes, forget how vital it is. What's wrong with Art that insists too severely on pressing people's faces against the miseries of actual existence is not that we shouldn't have to confront Darfur or Iraq, poverty or oppression; it's that such art rarely gives us the imaginative wiggle room to think of how things might be improved, or challenged, or even accepted. Imaginative wiggle room, on the other hand, is something SF/Fantasy is very good at.
It's a shame Richard Dawson's gone, because I can now imagine his voice barking out: "Top five epithets used to denigrate SF&F ... number one? 'ESCAPISM'!"
I've never understood how the "escapism" of SF&F is supposed to be sillier than, or less mature than, the "escapism" of golf, football matches, or getting tanked on vintage wines and wrecking cars with six-figure pricetags. Dig as you might, the only places you still find such an attitude upheld — let alone taken seriously — is in the minds of academics and critics who have an allegedly-embattled ivory tower to defend. Most people simply have no skin in this game anymore.
As Roberts hints at above, what we call "escapism" is really just a slightly derogatory way to describe imagination. And imagination, in turn, is about looking at life from something other than the same old angles and seeing what comes up.
I mentioned once before someone I know who couldn't read SF&F because, in her words, she had no imagination. I always found that point of view bizarre, because it's the author whose imagination is most pivotal in such work. I've since revised that viewpoint a little bit, and come to the understanding that it's about the space created between the author and the reader — which, in turn, means a reader who doesn't feel like they have much imagination to begin with won't feel like they're upholding their end of the bargain.
Now I'm starting to wonder if that was a case of someone who is simply that much more honest than most people are. Calling someone unimaginative works as an all-around insult, in a society that has convinced itself that imagination automatically equals genius. This is not to say that imagination isn't a desirable quantity — just that we confuse the mere having of it with the enacting of it. It's the difference between someone who talks about the great book they're going to write once they retire and the other someone who actually writes the damn thing by only taking a half-hour for lunch and spending the rest of the time scribbling. There's an imagination involved there which might go unnoticed: the imagination required to figure out how to make time where before there was none.
Back to the main subject. What it is about SF&F that we use to "escape" is not the mundane-ness of daily life itself. It is the way we look at that mundane-ness. It's the same thing that kids bring to bear when they see a sandbox: where everyone else just sees a pile of sand, they see raw material to build any number of things. I run the risk of sounding like one of those ghastly banking commercials that talks about "opportunity", but you get the idea. The more we get into the habit of not looking at our world with the same habitual blinders on, the less we see the same old things.
The danger here, again, is when we assume just having exposure to new ways of seeing is the same as having the sight itself. The grossest form of this conflation is in the guy who says he can write a novel because he's read so many of them already; at that rate, everyone who watches reruns of M*A*S*H is a combat triage expert. (Just stay out of the ER in the hospital I get taken to when I keel over, thanks.)
The less obvious version is when someone writes an SF/F work which on the surface of it has all the hallmarks of being "visionary" — as in, it shows strange and wonderful things happening — but the way those things are treated shows that the author has simply fallen back on the legacy of previous SF&F to do his seeing for him. That's why it helps to read more than just the generations of work that came before you in the same vein, since that provides you with new and varied ways of seeing. Why settle for the same old? Isn't it a lack of imagination to do so?