You make a good point about the ease of grading discussions in D2L and the seeming lack of ease of grading discussions in Moodle. I've asked some of the folks who designed Moodle forums why the forums were designed that way, but I've never gotten a really satisfying answer.
I suspect some of the apparent muddle of grading Moodle discussions stems from Moodle's social constructionist pedagogy, which posits that educators are supposed to care more about building knowledge through discussion (or "negotiation") than they are about transactional grading, which is something D2L really excels at.
I work with D2L and the other day an instructor said, "Hey! D2L has this polling feature my students and I can use to evaluate discussions!" I thought, Haven't we gone back to the future? But I had to admit I have never seen an instructor use this feature in D2L. Most instructors use discussions to assess student work and assign grades, and want the easiest and quickest way to do this, which is kinda sad, really.