Order of preference: Mac 32bit * 64bit .net --- below only theoretically interesting freebsd netbsd jvm
For my personal project I'm currently fine with only 32-bit x86 support. Mac support would definitely be the most important step here, especially since many people I know which would like to use D are on Macs and are discouraged by the bad support for that platform. Also it will be an important platform for the time when the project goes public.
At work I would like to eventually start introducing D when D2 has settled a bit (the planned threading model will be very interesting there). In that environment all of Win/32/64 and Mac/32/64 would have to be supported. 64-bit support can be very useful there, as virtual address space is a major optimization target (image processing and database operations). In addition, 64-bit support is required by marketing needs alone (people with their shiny new Vista-64 PC asking why the software does not use their hardware properly). Currently we also still have to support PPC. I'm not sure how long that requirement holds, probably depends on Apple's own deprecation policy.
.NET support would be very welcome if it could act as a transparent bridge between D and .NET (same would go for D and Cocoa) - This would be the part where Managed C++ (or Objective-C++) is currently used. If .net should be supported before OS X, this could also be used as a nice platform replacement.
