The SD is just the natural evolution of the Sigma. We've gone from the "F/C/V" to the "E" to the "VE" and now to the "SD". S&W finally gave the gun a good trigger and useable accessory rail, combined with a tritium front. Remember what S&W originally sought to do with Sigma - combine cutting-edge ergonomics, 3rd-gen DAO, and Glock-like simplicity. All previous efforts fell short of the mark, and the design ultimately failed in the Law Enforcement market. I think S&W may have something good here in the SD. It won't be a LE gun (the M&P IS the "Anti-Glock" that the Sigma and SW99 failed to be), but I think it will appeal to non-LE customers because of it's simplicity. They DID get the trigger right, and it feels (to me) like a SW6946.
I don't hold out much hope for a .45ACP version - they couldn't do it with the original Sigma (it started life as a .40S&W with 9mm scaled-in), and that design has been around for over a decade. I would REALLY love to see a .357Sig version of the SD.......