F. Brett Cox led off by saying that they would be taking "slipstream" beyond Bruce Sterling's original formulation -- composed largely of mainstream novelists writing books using genre SF elements.
John Kessel defined slipstream as fiction that creates cognitive dissonance due to having elements that cannot be reconciled.
Catherynne Valente commented that many things that other people considered slipstream, she considered just plain fantasy. Victoria McManus echoed this.<
Graham Sleight added that slipstream was by definition contemporary -- not necessarily in literal setting, but that it somehow reflected the strangeness of contemporary life, that it fails to make sense.
Theodora Goss spoke of the 20th century split between realistic fiction and fantasy, and the continuum that now exists between these two points. Slipstream is fiction in the middle, that thus creates a clash of reading protocols. The use of expected tropes doesn't lead to the expected clarity.
So, with the final comment we actually get somewhere interesting. For all my differences with Goss on the concept of interstitial that middle sentence is in pretty much total harmony with my own views on slipstream. The 20th Century split is more problematic though and I think it is this that causes Sleight to end up talking such cobblers on the subject. I don't think this continuum now exists, I think it has always existed and I don't think there is anything uniquely strange about contemporary life.
Is slipstream an inherently 20th Century literature? Well, maybe, but only in the same way science fiction is. That is to say yes as a labelled entity but no as an underlying literature. Sleight seems to slip into some sort of fallacy of equivocation where because he views slipstream as an essentially 20th Century literature then literature about the 20th Century somehow becomes slipstream. In this way he can co-opt not just Don DeLillo's wonderful post-modern catastrophe novel White Noise but also his historical biopsy Libra.
The "feeling very strange" conception of slipstream prioritises the post-modern aspects of Sterling's original essay to the expense of all other. In the end it estranges it from its much simpler core - "fiction in the middle" - and reduces it to books published in the last fifty years of interest to the literate SF fan.