1. You can build on top of a player. This can get him inside the unrendered part of the voxels, +leaving him stuck. The only ways out are suicide or waiting for someone to dig you out.
2. Looking straight down or up is jerky and adds charm to the game, but you can see every entity (players, intel, tent) through the block you're standing on/right over you. I've killed a few people using this AND was killed by a few people because of this.

This has already been noticed a very long time ago.

That is due to the game engine.

Personally I view these as positive aspects of the game.

How building on top of a player and looking through the floor/ceiling is a positive aspect? It's just stupid. You're digging and digging and wabam, you're dead because the enemy saw you through the floor.

While I surely forced a griefer disconnect by building him into the blocks (heh heh :D)... well, we just gotta make sure they don't find out.