So I build my nice particle torch flame and I want to use it in another scene. How do I do that?
You do not actually need to export the particle system to use it in another scene in the same project. All's you have to do is create a new empty prefab in the project window and drag your particle system into the prefab. The prefab will store all the information that your system had and you can drag it into any scene or place it on any inspector variable to use an exact replica of it.
If you want to use it in another project, You have to Assets/ExportPackage and add the particle system to the package (you might also have to make a prefab for it) and hit export.