I've been trying all week to come up with something profound to say about this one, and I find that I really don't have too much to add to what others have already said. It's just a great early episode. There's a real sense of crisis operating all through the segment, with an even more realistic feel than that of "The Corbomite Manuever". The final minutes of this show are as awesomely suspenseful and dramatically cathartic as anything in the original series. It's a shame that John Black only wrote this one episode for Star Trek.
And - oh yeah, the original musical score rules once again as well. I don't think I need to point out to anyone the greatness of the music - one quality which will never flag throughout the original series' full run. Unbelievably, I read a post by some idiot recently where he said if he had the chance he'd re-release the original series with a newly recorded modern score. Hard to fathom some people.