Glancing at the Wikipedia article, a few impressions:
The article looks like it was written by enthusiasts, so I rather question whether it has WP's Neutral Point of View.
It looks like it does some things better than C++ (which isn't really a hard goal), but it doesn't look to me like it does enough things better. Lots of people have worked on a better C++, and the only two candidates to succeed on a large scale have had massive corporate backing. There's now three languages in that niche: C++, Java, and C#. Squeezing another one in is not going to be easy.
The WP article said it didn't handle Unicode smoothly, which to me is a showstopper. I'm willing to forgive older languages for having problems with Unicode, but not newer ones.
I don't see why I'd want to learn D. I'm potentially interested in learning languages that are different from what I know, or lots better. D doesn't look like either. Moreover, I do my home stuff on Linux (or possibly OSX), and it looks like the main development is only for Windows (which I use at work, but only to play games on at home).
The primary development on Windows is a potential problem. Unless and until anybody can convince Microsoft to adopt it for .NET, which I see as very unlikely, it won't get mainstream status there. The Unix-side ecosystem has lots of developers, and more room for a new language to force its way in, much as C++, Perl, and Python did.
I could be wrong (every so often I remember my first impression of the Web, that it would never catch on), but I'd bet fairly heavily against D becoming mainstream. I'm sure it's a nice language, but I've seen lots of nice languages that never became mainstream. The current direction seems to be aimed at displacing the products of large corporations, rather than climbing around in the wainscoting to emerge triumphant.
I do have some suggestions for its proponents.
Fix any Unicode problems. Now. It really doesn't matter what you have to change and discard, or how many programs you break. No language with Unicode issues has a future any brighter than its present.
Keep the Unix-side development at least as current as the Windows-side. It's a lot easier to get mindshare in a world where nobody is fixated on Visual Studio and no company dominates, and critical mass is probably smaller there.
Get some significant application written in D. Anything that a large number of people would recognize would be good for a start. That way, it'll at least look like a serious language, and perceptions are critical in this game.