As far as I'm informed, you could just repeat a move until you get the desired effect, since there is no cooldown or mana. Are there limits to repeating a move, and if so what are they?
First off, all of edgerunner's answers are great. But I wanted to add some Dungeon World specifics:
Check p.19 and you'll see that 6- isn't "failure" - it's "trouble". The GM will say what happens and the player will mark XP. You are attaching non-DW simulationist ideas to DW mechanics by your supposition that 6- means "failure."
These principles can apply in all sorts of games, and have been used by GMs for years. If the PCs have to climb a fence, they're just going to keep trying until they succeed, right? So even in traditional games, many GMs will read "failed" rolls as a lack of some quality - not fast enough, not quietly enough, not without hurting themselves, etc., instead of just keeping them on the wrong side of the fence.
This is because failure is boring and stops moving the story forward. So you are correct, there is no plain-old failure in DW. It's not in the GM's agenda to make the PCs fail. There is no move for failure.
So the problem isn't that edgerunner's ideas are non-optimal, it's that your concept of what 6- means is wrong and that static failure doesn't exist in Dungeon World.
Expanding on 6-
From the text:
Generally when the players are just looking at you to find out what happens you make a soft move, otherwise you make a hard move.
Somewhere in Apocalypse World itself it says about hard moves:
make as hard and direct a move as you like
Early PbtA games like DW assumed you understood Apocalypse World. And this phrase is often tacitly implied in PbtA games even today.
6- means trouble as I said. The GM is free, on 6-, to make a move as hard as they like. That doesn't mean as hard as you can think of.
It’s not the meaner the better, although mean is often good. Best is: make it irrevocable.
So while a 7-9 should substantially give the character what they wanted (they accomplish their intent even if their action created complication), on 6- you are free to deny the intent (the action still has to have consequences beyond "no" though) and in addition make a move as hard and direct and irrevocable as you like.
Climbing a mountain a soft move is "The boulders above you on the rock face begin to wobble as the grappling hook you've tossed up there sets itself. What do you do?"
A harder move is "The boulders have tumbled off the edge of the ledge and after hanging nearly motionless for a tiny instant above you, are now plummeting towards you, gaining speed every moment. What do you do?"
A really hard move is "The boulders are yanked free by your grappling hook and come smashing into you, tearing you from your narrow perch and scattering the contents of your pack into the yawning emptiness beneath. What do you do?"
Nothing in Dungeon World is a straight conversion of D&D – everything is re-imagined. Even the base classes provided can't be used to convert a D&D character straight across (for example, in stock DW there's no way you can make a Dwarven Druid, while you can easily do so in D&D 3.x without creating a house rules). A straight conversion of new material is never going to be simple. To convert new material, you have to re-imagine it fresh, with the aim of capturing the flavour and style of the material instead of the raw abilities.
For complex or powerful races, you are actually better off creating a new playbook for the race, with a few optional abilities to pick depending on what class they are. So instead of picking up the Fighter book and choosing a Fighter-class Drow racial ability, you pick up the Drow book and pick a Drow-race Fighter class ability. Doing it this way you get a very Dungeon World–style character: they're the one Drow (or Warforged, or Dragonmarked, or …) in the party, and their abilities have a flavour and mix that is unique to them.
You still have the challenge of balancing a new playbook, but you avoid the mess that can easily result from hacking up and rebalancing the existing class books.