Many topics on this forum mention the STL format. While it is great that Slicer supports so many formats, I would advocate that Slicer discourage users from exporting to STL (e.g. from the “Save Scene and Unsaved Data” dialog change the file format pull-down to list “STL (.stl) deprecated” versus the current “STL (.stl)”. The only reason to export to STL is if it is the only format supported by the software or 3D printer you are using. Fortunately, modern 3D printing companies like Shapeways now support better formats such as OBJ.
The fundamental issue with STL is that it does not reuse vertices. This means that the resulting meshes will either look faceted (e.g. Slicer) or will be extremely slow to load (if the software attempts to unify vertices, e.g. Surfice). In addition, this feature makes this format exceptionally inefficient yielding large file sizes (about 3 times the file size of a simple uncompressed binary format). Not only are the faceted images ugly, they are slower and more resource intensive to render with modern methods.
The issue is illustrated in the image below. This is a mesh created with Slicer’s Editor function and then the same mesh is saved as both STL and OBJ format. The two meshes are then reloaded in Slicer. Note that the OBJ file has nice per-pixel shading, while the STL file has jagged per-triangle shading.
Given that the format is so popular with Slicer users, the other option is to support vertex-unification when STL files are loaded. This will make the loaded STL images look nicer (though loading times will increase).