Enterprise was designed to take the franchise into a new direction, unfortunately that direction meant the end of a franchise. The first problem with Enterprise was the characters, they were not particularly inspiring. First you have Captain Archer who often reminds me of a child who has found his dad's gun. Scott Bakula who plays the character seems to be unable to give Archer any sense of gravitas, charisma or respect other Star Trek Captains have. All he seems to be able to do is shout in a vain attempt to establish himself, and since the captain is the heart of any Star Trek show this is a major disappointment. Archer's crew consists of some of the dullest characters ever to appear in Star Trek, Mayweather the helmsman is a perfect example of everything dull. The only character who seems remotely interesting is Doctor Flox who actually seems to have a personality. When you compare characters in Enterprise to those in the next generation or Deep Space nine you realise how poorly conceived they are. The characters in DS9 are vibrant and interesting and are developed so well they make Enterprise characters look like permanent cardboard cutouts. This meant you never really cared about the characters regardless of the situation they were in.
Unfortunately the problems with the characters are only just the start. First of all we need to think about the writing, the concept itself could and should of worked in the right hands. Indeed for the first time a Star Trek show was running a continuously main story arc with minimal stand alone episodes, a step in the right direction.
The Problem was it had moved too far away from what Star Trek actually is. It was never about exploration, it was about conflict. It rarely presented you with morality stories that made you sit back and think, no, simply, it was not Star Trek anymore. The Temporal cold war story which spanned the first 3 seasons was gathering pace and was developed quite well. The Studio decided to axe this story because of the low ratings causing the show to take a nose dive into oblivion. The Studio probably did not realise that it was not the story which was turning people off it was the fact that the world of television had changed and left Enterprise behind the times, Enterprise was never innovative like Lost or 24, or cheap to make like American Idol. It was expensive and was not delivering. The final season was a hash of badly written and unconvincing story arcs achieving a whole new dimension of boredom. Never before have i been so bored watching Trek and finally i was glad it ended.