7. Poorly-Handled Serialization
For its third season, Enterprise did something unusually bold: they gave the whole season over to a single storyline. The show had not demonstrated that they could support serialization even close to this before, you see, for it's first three seasons, Enterprise was largely governed by a "Temporal Cold War." Apart from being confusing and poorly applied, this plot line dragged the show down merely because the show refused to abandoned it. Lots of televisions shows made this mistake when first experimenting with serialization. Heroes died on this principle, while Fringe decided to move on from it's early stories.
Enterprise was first among them though, and there for, it was their series to lose. And boy did they lose it. After three seasons of hacking away at a plot that was arguably only there to help excuse any problems with the rest of the Star Trek canon, they had to end it with alien Nazis. However, season three, in the long run, held together. At least the stories that were connected. What sabotaged it was its stand-alone episodes.