Enterprise was always criticised for it's large amount of action and violence, but was promoted as a more classic approach on the Star Trek universe.
Since this episode is completely placed in the dark mirror-universe, the show could play out all of it's darkness, action and violence without any contrictions, but still be true to the original, since the mirror-universe in Kirks time wasn a very enjoyable place either.
But the highpoint is, that this one tied a lot of classic element in, like the USS Defiant which disappeared in the tholian web on classic Star Trek, the redesign of the tholians and all the really cool little hints.
And of course there is the rebuild Enterprise / Defiant sets which were very enjoyable to be seen again.
Two thumbs up!