D isn't a good substitute for C due to its garbage collector.

Yes, people always chime in about how you can disable D's garbage collector, but then you're stuck with an awkward language that lacks support for basic things like exceptions, closures, strings, slices, and a host of other features not to mention pretty much any third party library.

If you are willing to accept a GC then D might be a good choice, if you find it fun then by all means go for it, but it's no longer so clear cut at that point given the wealth of GC'd programming languages available.