Unity "Importing within Itself" Flaw/Issue

I'll often create something or download some1's else's project that he or she was giving away 4 free. I want to use parts of my project or parts of the downloaded one 4 a different game/project. The problem is that when I import something it will often have something missing(materials, meshes, scripts, and so on). Finding out what these are and putting them where they belong is a pain. Is there a better way to solve this problem? If not then can we make a request of the people at Unity that they fix this issue?

Update: I recently became aware that "You can't just drag stuff from one project to another; you have to export and import packages so the metadata is kept intact. Without that, Unity has no way of knowing what is supposed to go with what."(thanks go to Eric5h5) but I would like an option to do it the former way anyway unless it is 100% impossible for Unity, the people at Unity, and a computer.

I don't know of any importing flaws. Sounds to me like whoever you're getting packages from isn't including all of the necessary items.

I have not found any way to do this, but if you would like to suggest this to them, you might want to post your idea at feedback.unity3d.com/.