That's called "lurking," and I consider it the correct default position in discussions of any sort--especially threaded ones. But you needn't lurk forever, and

That's called "lurking," and I consider it the correct default position in discussions of any sort--especially threaded ones. But you needn't lurk forever, and if you have something to add that isn't redundant or vacuous or--if you're particularly discerning--vapid, then you should feel welcome to comment. If you think of something clever, and someone else beats you to the punch, you should feel vindicated that a total stranger thought your idea was clever too. Beyond that, move on. Allow your clever surrogate's comment room to breathe, and if further discussion inspires some new idea in you, you can begin anew.