Hey, just finished playing. I didn't make it to Grub Heap, I landed in Moth Burrow and the game ended. I really liked the slow progression, and it wasn't boring because there was lots to think about, mainly the map, but also the balloon controls and the sort of "what's all this then" meta story.
I found it really easy to overshoot the 100 metre altitude brackets, I like the idea of some overshoot, but maybe a little less than what it is now. I found the compass / map tool quite fun to play with and liked deciphering the clues to work out which island was which on the map. I wasn't sure how exactly the directions related to the wind streams though, for instance with three points on the compass (left, right, and centre) does the wind stream pass through all of these or just the left and right points? If it passes through the centre then there are some weird v-shaped wind streams, alternately if it doesn't then there was some redundancy with multiple clues featuring the same start and end island.
I really liked the idea as a whole and could see myself really enjoying exploring the islands if they were a bit more developed. The sense of anticipation when you finally land on a new one is great. Picking up on what Zenuel said, perhaps some hook, or rope to pull you in to a designated landing spot when you get close to an island, there was a bit of walking to do when you land. Although more featured landscapes (hills, grass, trees) might help that too.