And what do you think their plan was?
From what I can remember, it was to go back in time, stop first contact, and assimilate Earth in the 21st Century. Lots of people have been explaining why this doesn't make sense for years, but I'll say it again now. Ever since the first time we saw the Borg we learned that they were primarily interested in acquiring new technology. So in their first attempt to assimilate Earth and the Federation they sent one ship. It got blown up when the Enterprise crew found their Achilles heel. So years later, in
First Contact, the Borg sent another lone ship to Earth, it got blown up. But it launched a Sphere that went back in time to a few days before first contact.
Now here are the things that don't make sense:
First, If the Borg have the ability to travel back in time why didn't they do that when they where still in the Delta Quadrant? Then travel unopposed to 21st century Earth and assimilate it.
Secondly, why would the Borg even be interested in assimilating 21st century Earth? They don't have any of the cool 24th Century technology that the Borg were originally interested in. Assuming they did assimilate Earth back in the 21st Century all they would have is one small insignificant planet thousands of light years from their base of power.
Thirdly, why would the Borg even think to stop First Contact? That indicated a degree of lateral thinking that the Borg were never shown to posses. They see someone with some piece of tech they want, so they take it. Nothing more complex than that. The Borg were remarkably single minded. That was part of their appeal. They were more of a force of nature than an evil race. They were kind of like the shark from
Jaws, they roamed the Galaxy consuming what they wanted. Nothing more.
Fourthly, whatever happened to the adaptive Borg we saw on TNG? The ones who would adapt to any weapons frequency or tactic that is used against them? The first time the Enterprise crew fought the Borg it took an act of Q to save them. The second time Data jacked into their Unimatrix and put them into Sleep mode. Presumably they fixed their Network settings to prevent that from happing again. All the while Starfleet Officers were free to walk around Borg Ships as long as they didn't pose a threat. In
First Contact the Borg could still adapt to weapons frequencies after they took a few hits. But they seemed to forget to adapt to any of Starfleet’s tactics. So even after showing how much of a threat that Starfleet Officers could be when they were poking around. The Borg still let Picard’s tactical teams get all the way to main Engineering, just because they kept their weapons down and didn't present them selves as an immediate threat.
I could go on, but like I said all of these points have been brought up by others over the years.
...and adding the Borg Queen were terrible.
Why shouldn't the Borg have an embodiment of the Collective mind?
After all, if you have billions of drones connected together, why shouldn't they function as one mind? Your brain cells do it easily enough. And why shouldn't the Queen be an individual, emoting all over the place? Your brain cells don't have any individuality and can't emote, but when they all act as one, they form your mind, don't they? A mind with a sense of individuality and emotions. So why should it be so implausible that the Borg do exactly the same thing?
If the Borg Queen had only been an embodiment of the collective, kind of like Locutus, that wouldn’t have been so bad. Instead it was clear in
First Contact, and
Voyager later on, that she was the
leader of the collective. That is a big difference. The problem with the Queen, was that the Borg were shown to not have a single leader. They were all of one mind. It was explicitly stated in
Q Who what the advantages of having no single leader were. Troi said that a single leader can make mistakes, but the combined whole is less likely to. In
First Contact and afterwards The Borg Queen was always making mistakes. The Borg were much scarier and a more effective villain before the Queen. Before, you couldn't out run, out fight, or even out think the Borg. After, it was pretty easy to out think or trick the Borg Queen.
The Borg Queen was just lazy writing. They could have kept what was unique to the Borg, not having a single leader, and had a really cool villain in the Borg collective. But they needed a bad guy for Picard to go Mano a Mano with (again), at the climax of the movie.
First Contact in general was just lazily written. I don't hate it though, I still say that it's easily the best TNG movie (not that that's saying much though). But Star Trek was once a show that took risks, it was creative, a lot of effort went into it's writing. But looking back, in hindsight,
First Contact was were it became obvious that, with the exception of DS9, the Writers were beginning to give up.