My initial reaction to the manifesto (Wikipedia, with useful links), which stirred up some controversy, was "You'll pry my FTL and aliens out of my cold dead fingers". And that was my initial reaction to Ryman's speech about it -- well, that and a tangential story idea that wouldn't be mundane SF at all.
But then it dawned on me. I'm not saying this as something I believe, I'm saying it to explain what I think Geoff Ryman was saying.
SF is becoming the work of the third artist. The first artist goes out and paints from life. The second artist copies the first artist. The third artist copies the second artist. (I've usually seen this analogy applies to fantasy, with Tolkien as the first artist.) The first artist put things in because there were there, or in the case of SF, because they were new cool speculation. The second artist put them in because they were trying to get close to the first. The third artist put them in because heck, that's what you put in. By the time you get to the third artist, using things like FTL and uploading yourself and aliens isn't speculating or asking "what if", it's playing with furniture in a doll's house. Going back to where we actually are and starting again, with the techniques but not the tropes of the genre, is trying to become a new first artist.
I'm sure that's what Geoff Ryman meant, and what that manifesto meant, and it makes sense even if you don't agree.
There's nothing wrong with entertainment for its own sake. But SF used to be something that made people think, rather than something comforting and familiar. Is SF becoming a genre in the way fantasy and mystery and romance are, where what you're getting is a variation on a theme? Kathy Morrow says for most people, most reading is comfort reading. I don't know if that's true, but it seems to me that the first reading of any SF novel isn't -- shouldn't be -- a comfort read. (Re-reading is different.)
The most interesting responses to all this came from my family. zorinth's immediate example of Mundane SF was Robert Charles Wilson's Spin, a book that would never have occurred to me, or probably to them for that matter, as an example. But it's set on Earth. The science in Spin is definitely not a piece of SF furniture, it's a new "what if". And it's using that "what if" to provide a timescale to do SFnal things like terraform Mars that wouldn't otherwise be realistic. Spin is a really interesting novel to consider in that context. Z says when he said he wanted more books like Spin and Permutation City what he meant was that he wanted more Mundane SF, except he didn't know about it. He asked if he could borrow Air. People talk a lot about getting teenagers to read SF. Maybe this is what they want. (OK, Z's 16 and he's been reading SF all his life. But I think it's relevant anyway, that he had such a strong response to this.)
Then when I reported all this to rysmiel and rezendi after coming home and making dinner, rysmiel pointed out that it was throwing away some babies in the bathwater by considering mature works that deliberately and consciously reference and build on other works of the genre, that are making something new by using one's assumptions about rearranging furniture, which is a very good point.
All the same, I have been worried for some time about the increasing trend for SF to give us futures we can't get to from here. I wrote a piece for the NYRoSF about this, in the context of John Barnes The Sky So Big And Black, and the trend is only increasing since, with the latest example being MacLeod's The Execution Channel. The reason it worries me is that while SF as a genre has always seemed to open out infinite possibilities for the future, this seems to be shutting us off from any thought of reaching it. When people wrote Venus as a swamp when they thought it was, that was a different thing from people doing it now.
I looked at the shelf over rezendi's shoulder and saw two books I'd enjoyed a great deal, David Weber's Off Armageddon Reef and Walter Jon Williams's Dread Empire's Fall series. Both of them are well written terrific reads which have to be seen as rearranging SF furniture. But right next to them were Williams's other books, (which are almost all "make you think" books) and, cheeringly, making me feel more optimistic about the whole genre, last year's Hugo winner, as adored by teenagers, Spin.