In the series finale of Star Trek: Voyager, we hear Lt. Barclay mention that there's a transwarp aperture forming about a light year from Earth. This was from the Borg's transwarp hub in the Delta Quadrant, and the Federation looked to be prepping for the Borg vessels that would come through.
My question is, if the Borg had the ability to do this all along, why didn't they do this much earlier, destroy the Federation, and be done with it?
From The Next Generation we do know that they destroyed the majority of the Federation fleet during the battle of Wolf 359. So doing something like that again wouldn't be a difficult task for the Borg.
It just doesn't seem very realistic that they would never use such an advantage against their foe, then again it's a TV show :).