[sowbug]

(We have descended into one of the deeper circles of grammar hell. I will remind you that you're free to leave at any time.)

Yes, exactly. English grammar actually doesn't require the "and" to end a list (leaving it out is called "asyndeton" if you're curious). A good example is Lincoln's Gettysburg Address: "... and that government of the people, by the people, for the people, shall not perish from the earth."

So after all this, there actually is a way to analyze the example that is strictly valid. But most people would look askance at the standalone sentence "This product is fast, lightweight." That is, I suppose, unless someone like Abraham Lincoln worked it into his next speech.

[MarkusQ]

Well, if we're going by what "most people would accept" we should probably allow:

* This product is fast/lightweight.

* This product is fast. Lightweight.

But yeah, this way lies madness. Unless we passed somewhere it in the dark back there.