How does parley finish a fight?
If I'm the GM and I say "So, the orc chieftan blows his horn, summoning his honor guard, and charges you!" and the Bard says "I Parley him and tell him to stand down" my response is always "How? There's a bloodthirsty orc running at you, why is he listening to you?"
If they have a chance to talk before a fight, sure, they might be able to avoid combat. Which is great, and likely to generate more problems, since its the GM who gets to choose if they hold a grudge. But remember, "to Do It, Do It." In the middle of a battle? That's a tough time to get an enemy to listen to your lies and/or reasonable offers.
Hack and Slashing a dragon follows a similar idea. If you can actually get up close to a dragon, with you blade unsheathed, you can swing it at the dragon just as easily as you could at a goblin. But getting that close to a dragon? That's tough. You've probably had to sneak and fight your way through traps, guards, and other challenges just to get here.
There's a very D&D instinct that rolls against bigger things need to be tougher, and that's not really true in DW. Things are tougher because you have to be in a fictional position to do them.
Take the old staple of presenting a case before a king in court. In D&D, the king probably has high stats in resisting persuasion, since the assumption is that anyone can make that roll. In DW, the king isn't harder to make the roll against, but getting the opportunity is tough. You'll probably have to make a number of bribes, maybe take some quests on the king's behalf, and present a worthy gift, just to have the chance.