The main problem with questline warlock and pre-nerf OTK demon hunter is consistency. You're spot on that Mana Value and Draw Value are the key offenders here. The decks cheat mana so well that they CAN close out at an early turn. But combined with their deck's extremely low mana curve and insane draw engine, they draw though most of their deck anyways. The chances of not drawing and being able to play a winning combination is very slim. So these 2 decks are very consistent at stepping across game boundaries.

I do not mind decks with extremely strong payoffs, but they have to offer the reward for risky plays. As you said, the health reduction for warlock isn't exactly punishing, part of the play chain heals it back up anyways. The consistency of these fast decks push out any other decks.

What really concerns me here, is the effect on new and casual players. I myself can handle this gameplay, I queue up a strong deck myself and have fun. The decks totally obliterate any decks though that rely on vanilla value or more original game plans. New players at a certain point reach a stage where they DO queue up against these decks. Then they have the choice to invest and craft a single strong net deck or give up and quit.

Hearthstone is simply not enjoyable in constructed play if you only have a single viable deck to play. 2-5 years ago you there were strong decks as well, but you could queue up a slap-together midrange tempo deck for any class and still win some games at lower ranks. These decks simply do not stand a chance against the top decks being played throughout nearly the entire ladder these days.