One of the main problems was, he was almost the sole compiler developer, and could hardly keep up with the tasks of maintaining 2 parallel branches and developing new ideas at the same time. People complained that they couldn't get involved as much as they wanted. It's understandable that many people thought that D didn't have a solid future with such uncertainties.
Nowadays, these problems are mostly overcome with a much better organization: there are several committers for the compiler, and several committers for the standard library. Phobos is the standard library, it's maturing, D2 has shown its strengths over D1 and the community is united again, because not only it is deeply involved with the design of the language and standard lib(through the m-l), it is also involved with the implementation of essential parts of it. 2011 has been a very good year for D, and I think that more than ever, the whole project feels like it's going in the right direction.
edit: I guess another reason D isn't gaining as much traction as it could is, it has been removed from the Alioth computer language shootout. For a language which is aimed at raw speed (and was brilliant at that when it was still on the shootout), it's a severe blow.