Hey there! :3

Before I start, I think it may be best for you to restart your plot, or at least consider changing huge amounts of it. It's very hectic as it is, but use the following review to help you with a new plot.

Firstly, your terrain. Some areas are a little too jagged and uninteresting. You have areas of terrain that cut right across, and it doesn't look very interesting. Make the shape of your terrain more random, along the lines of this:
http://www.reverieinteractive.com/BetaGames/Minecraft_Clone/MCCloneTech1.PNG
This is a little more complex than what I was aiming for, but it shows you the layers well. Some can be jagged, some can be smooth. You can make as many as those amounts of layers, or you can only make like 3 or 4, it's completely up to you. That image is justa  good demonstration of how you could make natural looking flatter terrain to build structures on!
You then have other areas of your plot where there's sudden cliff looking terrain (the ice). Try and make it more gradual, where the base of the cliff meets the ground, by adding more blocks to the base, and spreading it out some more.
Your floating island is too square. Make it look more like an island, making it gradually get thinner towards the bottom.

Secondly, organics. Your large tree in the middle is way too chunky for the leaves you've given it, and the height you've built it to. I think it needs some drastic weightloss, and also needs a looot more branches/roots.
Type 'Plutouthere' into YouTube and watch some of his tree tutorials; they're perfect for this!
Your smaller trees are pretty good, however they need a couple small roots at the bottom of the tree too, that's all!

Lastly, your structure. The walls have no depth, and the detail you've added is on the wrong layer, kind of. It helps to start with a frame for your building:
Simple frame:  https://imgur.com/ONsoRxh
More complex frame: https://imgur.com/20eZjP8
Using the logs on the outside layer like this adds simple depth and makes it much easier to detail!
Your roof should also over hang - it shouldn't stop on the same layer as the wall - it just doesn't look right.
I think the block choices are also a little strange. I wouldn't use such a dark coloured wood!

Good luck!
- Dutty :3