In "Star Trek: The Next Generation", Q introduces the Enterprise to the Borg. Up until then, this awful group of assimilators were never seen on the show before and they were a complete surprise to the crew. They also soon became huge and exciting enemies on the various other Trek shows. However, "Star Trek: Enterprise" is set something like 200 years BEFORE all this. So how can the crew have an encounter with the Borg in the 22nd century and yet no one in the 24th century knew about them?! Huh?! This just doesn't make any sense--especially since with computers and so much data on the Borg you wonder HOW this could be.
When the show begins, some frozen Borg are discovered by scientists. Amazingly, although they were frozen for a very, very long time, one of the seemingly dead Borg revives and begins doing what Borg do best--assimilating the crap out of everything. Soon these scientists are all Borg and when Enterprise comes to help, it is nearly destroyed by these jerks.
In addition to the inconsistency of even having the Borg in the show, it's very inconsistent how Dr. Phlox is assimilated. Normally, within seconds a being becomes a full-fledge Borg drone. Yet, inexplicably, his transformation is VERY slow and is later easily reversed! Again, the correct response is...huh?!
Despite all these annoying errors that violate canon, the show is reasonably exciting. For folks who don't care about these problems, I'd give this one a 7. For Trek fans, I'd give this one a 2 at best.