Well, other people are saying what I want to say about V feeling guilty NOT ONLY about Familicide, but also about vir pride that presumably let to accepting the splice (even though in reality it didn't).
So I'm going to focus on another interesting point.
A line of explanation is still needed. "I rebuked your wights over to my team" and "I gained a level so I have this spell now" is just as much explanation as "Oh, and have I mentioned I can shoot lasers from my eyes?". It is needed because not all of the audience is familiar with DnD. See more below.
That may be a bit of overstatement, but yeah, I basically agree.
What I think is generally bad about basing a comic on D&D is that all powers become foreseeable. Not entirely predictable, you never know what spells a wizard has in his spellbook and which of them he has prepared today (unless you do), but you can plan for every power-related contingency. All surprises that are left are about people's decisions and relationships, which kinda defeats the point (or at least part of the point) of using the fantasy magic system.
When a villain says "I'm in control of your undead army now!" the audience and characters don't go "Oh God! I had no idea he could do that!", they go "Well duh, he's an Evil cleric, should have planned better". I'm not saying it's necessarily bad, just that all potential benefits by letting the audience know the system beforehand are outweighed by the drawbacks more or less equally.
However, the argument "you don't have to develop the system from scratch" is, from creative point of view, far outweighed by "you don't get to develop the system from scratch". Rich has already said why - because you have to tailor the plot to the powers, not the powers to the plot, and... and don't get to develop a new system from scratch, duh. (although that can be just me)
When you are writing your own system, you have to care to make all your powers look logical from a certain point of view (examples: "Duh, redhead with a violent temper and a lighter in her pocket, it's only natural that she would control fire", "Well, she is mysterious and with unknown origins, no wonder she has some mystical powers"), but as a result, they REALLY look awesome. People say that Roy is awesome for being able to beat an army, but honestly... their calculations show that at his level he SHOULD be able to beat an army. It's predictable. It's what everyone of his level gets. We know for certain that Tarquin would be able to beat an even bigger army because he's of the same class (presumably) and higher level than Roy. The closest thing to dramatic personal empowering we've seen was Elan seeing through the illusion, and it was soon mitigated by showing Nale do the same.
Because the whole point of D&D rules is consistency and equality: if one person can do that, another of the same class and level can do it too.
You don't need to have an undeveloped and unexplained magic system to avoid that. You merely need to incorporate inpredictability into magic system, allowing for some to be more awesome than others because of personal resolve (translated to D&D as will saves), learning capability (translated to D&D as earning XP according to strict rules) and perhaps just personality.
And about the benefits of the audience already being familiar with the system... but not all of it is. There are lots of people in the forums, and probably even more out there, who have never heard the term "DnD" before starting to read the comic. For some of them, Roy cleaving through the army is awesome because they didn't see that coming, for some - it's unrealistic and feels like cheating, because DnD isn't realistic enough to provide drama for those who are not used to it.
I hope I was coherent enough.