best thing about Snow Leopard so far: It restores per-application persistence for input method settings, i. e. I can have a Japanese keyboard set for my dictionary app and an English one set in the document I'm typing my translation into, and when I cmd-tab between the two processes the input method changes accordingly. This functionality was removed in Leopard, and I am unutterably glad it is back.

the worst thing about Snow Leopard so far: it breaks my awesome Hal 9000 screensaver, which apparently will remain broken until Adobe produces a 64-bit version of Flash (which is to say, never ever ever.)