In the book business the books that seem to last the longest are the ones picked for school reading lists, are short, and use time neutral language to avoid being quickly "dated".
I'm quite dubious about that assertion, given the history of what has been picked by the schools as opposed to what's survived for any great length of time. Lovecraft, for instance, is only now being introduced into school curriculums (with perhaps two or three exceptions), yet 70 years after his death (as of March) his work has become more popular, and more widely respected (and more relevant) than ever. L. Frank Baum's Oz books were considered intolerably "low-brow" and were scorned by teachers, librarians, and parent organizations alike... yet they've hung on to become American classics a century and more after they began. Nathaniel Hawthorne was very slow in being recognized by educators in this country, and Poe took nearly a half-century after his demise before he was being included in any standard reading programs for schools. Shakespeare underwent a decline of nearly a century after his death, as well. So I seriously question that what the schools pick has much to do with what lasts as long as a century ... it tends to be much more difficult than that; it's something that appeals because it resonates on a deep emotional note with readers through ages; and that's a very difficult thing to define.
As for contemporary writers who'll be read that far down the line... that's extremely difficult to say (I'd actually say darn near impossible; we just don't have enough perspective). We can bet it'll have little to do with what sells well in our own time, though; look at those writers throughout publishing that have sold very well in their own time... with very, very few exceptions, they are all forgotten (Francis Lathom, anyone? Eleanor Sleath?) But I'll take a stab at it, anyway...
I'd say that some of Asimov will make it; after all,
I, Robot,
Pebble in the Sky,
The Caves of Steel, etc., have made it past the half-way mark. (But that may be a bit too far back, given the 30-year specification.) Still, I think some of his later work will survive, as well, though whether it will be in the sf/f field ... who knows?
Heinlein, I'm pretty certain will, as he has and will continue to stir controversy... and his books are, if nothing else, quite good for getting people to think about things; even his later work has much to recommend it, didactic though it can often be. In fact, in the long run, that may actually be to its advantage, as much of the best literature over time has had a strong didactic streak.
I'd say Harlan Ellison has quite a few stories that will make the grade; more his sf/f than his mainstream, which is very much of its time (especially in his earlier work -- the later stories not so badly), so the referents date rather severely. Most likely a collection of his better tales that has not yet been done, which leaves behind the lesser works... that will continue to survive because of the emotional immediacy and the skill of the writing itself, which varies from verbal assault to pensive subtlety.
Actually, I think one of those who is likeliest to survive is Mieville. His work is so original in impact and imagery, that I'd give him rather good odds on lasting.
Moorcock: I would definitely expect some of his work to last, but others ... no.
Le Guin -- You may be right about
A Wizard of Earthsea... YA books are especially hard to peg, but as they've lasted this long, it's a good possibility. I think, perhaps,
The Left Hand of Darkness and
The Dispossessed, as well... maybe some of her others
And I think perhaps C. J. Cherryh will have some that make it, as well...
There are probably others, but those are the ones that come to me off the top of my head. In general, I think that the genres will survive much better than the mainstream, and probably sf and f best of all, because they are more akin to legend and myth (even if told in futuristic or very modern terms) and therefore tend to cross generational boundaries, where their clunkiness (in the newer generations' eyes) can be forgiven for the energy and the insight that transcends the momentary that bogs down so much of the mainstream itself.
