Well, the main one is that is in the group of continuation based frameworks, like other advanced web frameworks such are seaside (smalltalk) ocsigen (ocaml) or coccoon (JavaScript) . That means that it is not based on the event model, and the flow is in a single procedure. This makes the navigation much more readable, as a console application. But WASH is not continuation based, but log based, and the log was stored in the client, that means that it was also restful to a certain level. That approach solved the big problems of memory usage and serialization of continuation based frameworks.
It also solved the problem of form safety before the development of formlets and the widespread of applicative functors.
But I do not pretend to resucitate WASH. I just miss a continuation of this line of work , but this was not the case.
I took a deeper look at WASH after developping MFlow, years after the discontinuation of WASH. MFlow coincidentally, is log based in a different way. I supposed that it was based on continuations, and I didn't like them, but it was not. If the project would have been prosecuted, who knows...It had a edge over other web developments in other languages, but it was hard to understand, just because the approach was original and unique.
I consider MFlow as a continuation of WASH in "philosophical" terms.