^ Your massive image is distorting the web page. Can you please resize it and provide a link to the larger scale? Thank you.
The Borg as a faceless entity, a mass collection of minds that you couldn't see, made it a very unsettling enemy. That an away team could beam over and the drones would essentially ignore them, because they're "too weak" to be bothered with, was just unprecedented.
In First Contact and in Voyager, they changed the Borg. It was no longer the faceless enemy. The introduction of the Queen turned them into something different. I could understand if they created another "liason", a drone who would basically speak for the Borg but in an unemotional, robotic way... this would work. But the Queen behaves like a human being with full emotion. And then, you have these nearly indestructible Borg cube ships where ONE cube took out 40 starships at Wolf359 as the initial premise, only to change whereby a single vessel like Voyager can stand up to one of them. They "weakened" somehow... it wasn't explained. There was no mention of Borg specific weapons being used that had remarkable improvements over previous weapons. And lastly, you have the assimilation thing. In TNG, they assimilated technology but destroyed humanoids. They learned how to adapt Picard into Borg form. Perhaps this could be considered their realization that assimilation is possible. But the nanoprobes as a means of assimilation was a huge leap. There wasn't any explanation for it. Also... the time it would take for someone to become assimilated varied a lot. In some cases, you see it happen within a minute, in others it takes hours.
I preferred the TNG Borg. They were more threatening, menacing, insidious... I liked the assimilation idea, but wish they'd have introduced it differently. That it would be more selective, not focused on assimilating everyone found. And done in a more believable way... that it was more of a hijacking of the brain functions, instead of "rewriting DNA". I really never liked how carelessly Star Trek treated DNA manipulation.