Why does Unity take Blender Models and screw them up?

I was wondering why this kept happening to me with Unity. I made a model in Blender than imported it to Unity but it doesn’t work how I want it to. Pictures are at the bottom for visuals.

Miscalculated normals and Non-manifold meshes… in a nutshell

If you make a hollow object which has walls made of 1 face then, if the normal is calculated outside, when viewed from the inside the face will disappear. If you want to have a mesh wall visible from both sides, make it double faced (solidify it). You can have double sided normals as an option in Blender but I’m unsure if Unity handles it.

Non-manifold meshes are those which are ‘non-watertight’, ie it has holes in it, or somehow the mesh is inconsistent and non-fluid. In edit mode select Vertex mode, deselect your mesh (A twice) then in Select menu click Non-Manifold. It will highlight parts of the mesh which are in question. Such things you are looking for are: Internal faces or edges, disconnected vertices inside the mesh, vertices at the same point which are not merged creating a hold in your mesh, holes in your mesh.