OK people... I had a brainstorm while I was at work today, and I realized what separates the standard ST universe from the Mirror universe, and its so simple!! It all ties back to a causality paradox that didn't happen in the parallel universe. Incase you're not familiar with a causality paradox, it means that someone or something from the future, travels into the past, interacts with that past in a way that you would expect to change things, but in actuality changes things to what they were supposed to be. However, if that event in the past is crucial to lead up to the point in the future when the time traveller departed from, why was it already there in the past. Basically, something like that shouldn't be allowed to exist, but it does. Enough for theory, on to the specific events.
Since its all cyclical, its difficult to pick a point to start in, but I'll try. I'll start in ST:TNG season 3, the Borg attack Earth and fail. Then, during ST:FC, they try again, Borg ship destroyed but it launches the sphere which travels back in time, followed by the Enterprise. The Borg ship fires on Cochrane's launch site, but is itself destroyed by the Enterprise, then plumets through the atmosphere and lands in the frozen wasteland of Antarctica. Members of the Enterprise crew beam down and eventually reveal the truth to Cochrane and in the events that follow, make him the man that Federation history records him to be. Then, during Enterprise season 2, the Borg from the sphere are revived, assimalate people, steal a ship and ultimately manage to send a slow-speed transmission to the rest of the collective. The time of arrival and their expect response time prompt the Borg to send an exploratory mission to the Neutral zone where they attack and sample Federation and Romulan technology as mentioned in the TNG season 2 prmiere. If not for that transmission, they woudn't have come and they wouldn't have attempted invasions twice, wouldn't have gone back in time and wouldn't have interfered with First Contact. As I said above, no one of these events is the trigger, and by all rights this causality paradox shouldn't exist, but it does... however in the Mirror universe, this paradox doesn't exist.
In the Mirror Universe, officers from the Enterprise-E weren't present to influence Cochrane, n'or were the Borg there to attack the facility. Because the Borg never came, there was no leftovers from their sphere to awaken and contact the Collective during season 2 of Enterprise. Cochrane, not having been enlightened by his time with Riker, Troy, and the rest, not having been given a glimpse of the future that he would help create, was more concerned with protecting what he'd made, and so he pulled out a shotgun, and blew away the Vulcan. From then on, we have a different series of events, all precipitated by the lack of a certain causality paradox. After all, if a causality paradox shouldn't be allowed to exist, whose to say that the laws of temporal physics in the mirror universe aren't such that such a paradox can't exist and as such doesn't. No signal was sent to lure the Borg to our side of the galaxy, and by the time Q would have sent the Enterprise to that side of the Galaxy during TNG season 2, the Empire had been overwhelmed and crushed, and the human race was enslaved, as per the DS9 episode late in Season 2. The Borg never came, so the Mirror universe became what we've seen. Their only influence is in contact with people and technology from the Federation side, starting with the current Enterprise episode, followed by the TOS episode "Mirror, Mirror", then on to a series of episodes on DS9, all because the Borg never came, in either the future or the past.
Well, that's it. That's the root difference.
Comments?