I've only had one session with the druid player and he's already figured out how he can circumvent most any physical obstacle. He doesn't even know the rules, so I have a hard time labeling what he's doing "power-gaming".
What does the player of the druid want out of the game? Ways to circumvent any physical obstacle? If he has 'figured this out' without making any moves or engaing with the mechanics (ie: just authoring stories together), then sure... But your examples suggest that they
are driving to 'work the system' (at least with an understanding of the Druid's shapeshifting moves) in order to overcome antagonism? This is creative, and quite synonomous with old school D&D play and a thing to be celebrated!
But remember, you are a fan of the characters yeah? It isn't you Vs the party. Your job is to fill their lives full of adventure, to portray a fantastic world and play to find out what happens. It sounds like you are doing just that :) Just logically extend this beyond the character's immediate situation.
Think offscreen too. Is the Druid player feeling like their flags are being met? Are they having fun? Is there enough adventure in their game? If so, well and good. Play on :)
If its tasting a bit sour because you feel your obstacles are being overcome with little challenge and the (over) use of a particular move.... Change your obstacles, that is your purview as GM after all.
Remember too that you can make as hard a move as you'd like on a miss, and it doesn't always have to be directed at the player's character you failed the roll.
Think dangerous, always. Shapeshifting into flies when there are flyswatting creatures around is very dangerous methinks.
So this is Great! Just make your moves as GM and use all of the wonderful suggestions given in this thread so far, James especially has given you plenty of ways to incorporate the consequences of developing this canon in
your Dungeon World (using the rules as written).
Don't underestimate the power of
asking provocative questions before the move is made. Nut out the why and the how and the consequences. Dig deep and encourage the player's to author their own suggestions into the reality of your world. Then the fiction empowers your (group's) interpretation of the move.
That is the beauty of this system. Every group, every individual game develops its own fictional rules that frame the moves and support their narrative expression in ways unique to that particular character and story.
Your group's druid's fictional iteration will be very different to mine and that's OK (and encouraged by the rules!)