I watched this episode again last night. My first repeat viewing of an Enterprise hour since The Aenar in February, which I watched mostly for Jeffrey Combs’s performance.
Gotta say, it doesn’t entirely hold up on a second go-round. Lots of plot holes and stuff to nitpick. (Example: The Tholian is beamed off its doomed ship into decon. Archer then tells Phlox to adjust the environment for the Tholian’s needs. Phlox does this, raising the temperature and so on. But then later, they torture it by making the room colder, and Phlox kills it with a steep drop, causing the Tholian to asplode. Okay, but the Tholian was in the decon chamber in human-normal conditions for at least a minute following the beamout. How come it didn’t asplode right away?)
But nitpicks aside, I legitimately enjoyed this hour of Trek. The quality that has been missing for me, the thing I’ve been harping on the last four seasons, is the lack of interpersonal differentiation, and the lack of any sense that the characters are driving the story instead of the story driving the characters. I called them action figures at least once and probably more than that; it’s what happens when the plot is master. The Orion slave girl episode was a perfect example: It didn’t matter who the characters were; the plot required them to behave a certain way, and so they did.
In this hour, on the other hand, the story is driven by the characters’ choices, by what they want from one another and how they go about getting it. And I gotta say, it made all the difference.
Now I’ll admit, the first time I watched this hour, I probably liked it more than it deserved, just because I was so hungry for interpersonal dynamics that the fact that there were some was enough for me. On second viewing, there was some unnecessary stuff, and some narrative missteps. F’rinstance, I didn’t need T’Pol to provide Trip with the particulars of how she manipulated him into causing the overload; it would have been plenty for him to realize that she had done something, yell at her, and have her simply cock an eyebrow and walk away. We can fill in the gaps; having the detailed explanation laid out like that smacks of the tongue-in-cheek verbosity of the villains on the old Batman TV show.
But the simple fact that each character was different and each character had a different objective and each character took action in pursuit of those objectives was amazingly fun to see in latter-day Trek. Definitely a far cry from the hive mind of saints and lily-white heroes we’ve been getting, like a benevolent Borg, where everybody agrees on the objective and the methods and they just have to execute the consensus. That’s dull. This, on the other hand, was fun.
So like others have said, I would totally, totally enjoy seeing this stuff week after week, if they decided to do a show set partly or entirely in the mirror universe. I mean, I know, it wouldn’t be so eventful, with so much stuff happening, so many ships blowing up, so many characters getting killed, because you’d simply run out of cast members halfway through the season. 
But the personal dynamic is great. I love the spin on the rules of Trek storytelling, and I’d happily tune in if I could get something like this episode on a regular basis.
One thing I’d change, though: I’m not in love with the style, the hyper-stylized near-Shakespearean formality of the acting and the language and stuff. I’m not suggesting they should go all the way to Galactica-esque verite, but as I was watching, I was paying attention to the script and considering how the lines would sound and how the dialogue would work with a somewhat less, y’know, fussy delivery. (On the subject of which, I agree that Bakula came off worst trying to pull off the mirror-world grit, with, unfortunately, Linda Park a close second. I just didn’t buy the Machiavellian depths in either of them that were present in the rest of the crew. Billingsley’s Phlox was great, and I thought Montgomery’s cool-as-a-cucumber very deliberate “aye Captain” was a neat little acting choice, making a lot out of not very much dialogue.)
And as I considered the script, as I imagined how it would feel if presented with a somewhat more naturalistic style, I decided that it’s actually pretty good. There’s some really decent writing there. I’m just really getting tired of the cheesy Shatnerian style, y’know? It’s annoying enough in the normal show, but here in the mirrorverse, I don’t need the acting style to tell me that a character is evil. I don’t need Iago-esque sneering and diabolical revelry to identify the bad guy; I can draw my own conclusions from the character’s actions. I mean, wouldn’t it be more chilling if Archer were standing in front of Trip in the torture tube, and said “You’re working for the Admiral, aren’t you? What have you told him?” and then “Break him” in a casually professional, totally matter-of-fact tone instead of grinding and chewing his way through the dialogue like it’s bitter Klingon jerky?
So the point is, if there were a mirror-universe series with double dealings and deceit and betrayal and legitimate uncertainty, unpredictability, and danger, except presented with a somewhat more modern acting and production style, I wouldn’t hesitate to watch it.
Oh, and re the implausible similarities of the parallel universes, I haven’t read the Mirror books mentioned by others, but the symmetry rule mentioned by Fiver actually gives rise to a possible long-term story arc for a hypothetical future show. If there’s some connection between the two universes that forces events and people to remain linked, doesn’t that imply that one or the other is “in charge,” so to speak? Wouldn’t it be provocative for the Evil Twin universe to discover this rule, and find that the arbitrariness of certain events was due to their being dragged around, causally, in order to stay somewhat in sync with the “primary” universe? In DS9, for example, this is why Evil Worf found himself unexpectedly drawn toward Bajor, and here in Enterprise, Evil Forrest has to die on his ship to maintain balance with the death of Forrest in the “normal” universe. That, I think, would make a potentially interesting season-long story, or multi-season story, a cold (or even hot) war with the mirror universe as they discover that they’re not entirely the masters of their own fate, and they decide to attempt to either take over the “good” universe or sever the causal connection entirely. It would certainly be better than the idiotic “temporal cold war” Enterprise couldn’t make fly despite three long years of lethargically flogging the idea.
Definitely looking forward to part two. Please, Manny, let it be as cool as the first half.