First, some appearance things.
The island has one layer of grass atop plain stone. It'd look much better if there were a few layers of dirt beneath the grass, like most floating islands have.
Also, the island is a square. It'd look much more natural if it were an abstract shape.
The leaf formation on the tree is very strange. It hangs very far down on one side and is lopsided at the top.
You should mix up the flowers. Right now you only have the default bonemeal red and yellow flowers, which is almost a sign of laziness. Remove some of those and add in other flowers. Purple ones, blue ones, white ones, orange ones, etc.
The white carpet fences at the end look slightly out of place. You could change them to green wool to represent leaves, and instead of fences underneath, put string.
Try making the underneath of the island not just stone, but also a mix of cobble and andesite.
The first lava room's lava is not still. Making it still would look better.
The last lava room looks much too man-made. It'd be great if you made it look like a regular, natural spawning minecraft cave.
Parkour things. There were more problems with the appearance of the map.
Too many roofed jumps for my taste, especially in a medium map.
In the first white carpet fence jump after the tree, some players might hit their head on a fence above. Your best bet in fixing this is moving the above fence to the side or something.