The biggest disadvantage of D compared to Rust is that it does not have the kind of safety perspective that Rust does, and in particular does not provide safe constructs for concurrency.
Pretty sure immutable, purity, and thread-local statics are all safe constructs for concurrency; not to mention all the library features.
Rust probably is safer by some metric, but all those pointer types add considerable complexity.
The other argument against using D is that it has been around more than 10 years now, without much adoption and appears to be more likely on its way out rather than increasing popularity.
This is just false. Any metric you look at suggests D use is on the increase, and certainly starting to get more commercial interest.
It's worth noting that many languages take a long time before they blossom. It took Ruby 10 years before Rails appeared.
