Considering wall thickness and support
I do have to admit that slicing software has come a long way in the past few years. I remember when I started 3D printing, the software I was using was barely able to do much more than generate support for overhanging geometry. Nowadays, it can repair meshes, automatically orient models, and, in some cases, it can even print non-manifold geometry.
While I am all for taking advantage of this software when it makes sense, there are some things that you may want to control yourself, as you create your model, such as creating your model with proper wall thickness and creating geometry that will reduce the need for support.
Modeling wall thickness
Let me start by describing exactly what I am talking about here. I am using the word wall in reference to any geometry that creates the outside geometry of your model. To be properly manifold and printable, walls must have some thickness. They cannot exist as simple faces. Take the example in Figure...