First of all, it's not an unlimited resource; the Druid can only wild shape twice per short rest, limiting the amount of poison the Rogue can attempt to harvest and by RAW:
DMG p.258
Serpent Venom (Injury). This poison must be harvested from a dead or incapacitated poisonous snake.
Unless the Druid can retain his form after dying, the Rogue won't be able to farm Druid snake venom. Furthermore, the DMG says that harvesting poison requires a check:
DMG p.258
Crafting and Harvesting Poison
The creature must be incapacitated or dead, and the harvesting requires 1d6 minutes followed by a DC 20 Intelligence (Nature) check.
It goes on about how to add proficiency to it and what happens on a failure but the DMG at least says that harvesting parts from creatures is not an easy task (DC 20 is hard, after all)
The point I'm trying to make is that you should allow your players to do clever things but limit the powergamey-ness to a minimum level that doesn't completely break your game. How you do this is up to you, I personally find that, at my table, at least, that doing the "yes, but..." approach to GMing maximizes the fun. "Yes, but you have to make a check to see if you can get enough venom for a single dose (this is important, you can get the vial half full but that won't cut it!). The druid can help you by giving you Advantage on the check, but it's not automatic." Being flexible like this allows your PCs to at least attempt the thing they really wanna do, and fosters a fun game.
As for other possible attempts to exploit "infinite" things, allow and disallow at your own discretion. A good tip for this is to think about how overpowered it would be to allow it.
Take your infinite arrow feathers example, by pulling the feathers off the wild shaped druid (ouch!), he is able to save a whopping 1 GP (What a bargain!). Consequences may or may not exist, depending on the kind of game you're playing but I would personally rule that once the druid reverts, he finds that he lost some of his hair.
Use the Treesinger druid archetype.
In the Advanced Race Guide there's a racial archetype for elves that lets a druid do exactly this. It introduces four plant-companion options:
- Carnivorous Flower
- Crawling Vine
- Puffball
- Sapling Treant
If you're looking for a creature the party can come across in the wild, you could just use the stats for these companions (possibly increasing their effective druid level based on the party's APL).
Best Answer
No, by default. Your average Dungeon World dragon is an intelligent being like (or moreso) any human and doesn't count as an animal. The Druid is very much about nature and natural animals, not magical beasts or intelligent species.
But, that's by default. In Dungeon World, fiction rules all and in your group's Dungeon World you may have established that dragons are animals. Then it's fine and fitting that the Druid would know them as they know hawks and bears.
But if you haven't established that, say "no" to shapeshifting into a dragon—it would just cheapen the whole fictional underpinning of the move, and generating good fiction is the only reason the moves exist.
If, in the end, you do decide dragons are legit animals, I have one piece of advice: when the Druid misses on a shapeshift roll, make your move very hard. That's how the immense power of the move is balanced—riskier the form, the worse the consequences of a miss. If you pull your punches there, you'll end up with the Druid stealing everyone's spotlight and be a sad GM.