Is there anything wrong with saying: "All the spaghetti is gone." Or do you need to say: "All of the spaghetti is gone." Is there a rule that governs this construction? Is the "of" needed?
IMHO, "all the spaghetti is gone" is the right one. The expression "all of" means "no less or smaller than". For example: "she's all of forty years old, no doubt".

Hope this helps! Emotion: smile
Excellent reply, Raul, I couldn't have said it better myself!

Side note for the question: "All of" and "all" is one of the least enforced rules in the English world, so you'll commonly see "all of" and "all" in various places in print, depending on the author; it's best not to analyze it every time you see it, although you should try to use it correctly in your own writing.