For those who had developed applications with D,
I use Tango, Jive stacktrace, dconstructor, dunit, and selfmock. I maintain the last three of those, though.
Tango has reasonable documentation. I've run into a few places where it had less documentation than I needed; in these cases, I've made tickets requesting better documentation. Jive doesn't require documentation. I hope the others I use have reasonable documentation; I've been working on that a fair bit.
Yes, I use Tango.
You can create large applications in FORTRAN; D is better. The main issue is library support. Things like cryptography aren't well supported in D, as far as I know. But it's been pretty easy to find everything that I have needed. Anything else, you can probably write a wrapper around an existing library in C without much trouble.
I usually use vim, but now I'm switching to Descent. Its autocompletion is far better than Vim's.