I don't know why I'm surprised when Warren Ellis drags out his critical skills. I suppose it's because his default mode is so often dismissive.
Still, while we do get the admission that "Musical comedy makes [his] balls itch", he's more than willing to take on Dr. Horrible on its own terms and makes an interesting point about how hubris often derails inexperienced creators: Everyone wants to write some epic-length piece with tons of Deeper Meaning, and forgets that–to take a recent successful example–the first Harry Potter book was short and self-contained.
So release early, release often can also be a mantra for creators of content, not just software.