John's got it pretty much exactly right. The bokeh is somewhat better, but not a lot. The focusing on the 1.4D is barely faster than the 1.8D, but almost trivially so. (If you weren't paying attention, you'd never notice of someone switched them on you.) The 1.4 AFS is actually slower than the 1.8 AFS, and may even be slower than the 1.8 AFD. If the 50/f1.4 AFD is sharper than the 50/f1.8 AFD, it is only by a small margin. I doubt I would notice which one captured a given file if I were just browsing through them. I'm not even sure that I'd notice if I were pixel peeping, to be honest. They just aren't that far apart.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!