This is getting into a fuzzy area where one person's big difference is another person's small difference. I'd consider D templates to be a small difference from C++ templates in terms of the language[1] but big enough in terms of productivity that it would make sense to start a new project in D over C++ for that reason alone.
[1] A random commenter on HN might well conclude that there's no benefit to D's templates without using them.
When switching is costly, the potential upside must be big. When the other language seems similar, the upside also seems limited. When there are few differences, you can rationalize staying with the old language, because it may catch up with the features, or you can emulate the missing features, or just live without them, if the differences are small.