Instead of rehashing the same opinions and going where everybody's gone before, I will add a few of my own thoughts on the series. There are some spoilers ahead but since the series had run its course I will only give this sparse warning.
It's obvious Roddenberry's concept was fascinating as we can see in s1. What makes me wonder is just what WAS the original concept for the following seasons? What elements followed his concept and where did the show depart? It would seem that the concept of the Taelons' desire to use humanity to help them defend against the Jaridians was part of the original concept. So was the concept that humans had the potential to help but were not considered sufficiently evolved; the tension arising from the Taelons' differing views on what to do about it. It would also seem the ultimate resolution to this conflict was to be the joining of the Taelons and Jaridians with the help of the humans.
But what went awry? How detailed were Roddenberry's notes and where were the writers left on their own? What I noticed in general is that the series lacked CONTINUITY. One gets the feeling the writers were constantly changing their minds.
As has been noted, cast changes, starting with Boone's death, did not help keep the continuity. Liam's character was an example of this changing of direction and concept. First, his appearance was too "magical". He was too superhuman, in contrast to Boone's very human character. OK, fine. If you're going to have a superhuman character; then let him play the role and let the 'human' drama play around him. Yet suddenly we see Liam, this 'chosen being', acting more and more like an ordinary human. A change of direction. Then it's obvious the writers change their mind again and try to humanize him. They later even made him 'completely human', which completely contradicted his original concept. It became obvious the writers were changing their minds about what they wanted him to be. His character changes were symptomatic of the overall treatment of the series as it progressed.
Much of the overall concept and 'feel' seemed to keep changing as well. Most drastically in s5 when it's painfully obvious this was an ad hoc plot extension. Possibly because the show was supposed to end at the end of s4 but then they decided to stretch it out another season. Sorry - didn't work. s5 was an entirely different show from s1.
I don't believe I've ever seen a show decline quite the way this one did. I don't even consider s5 as part of the original concept... but even s1-s4 seemed to lose it after the original season. Which is too bad as it could have been one of the most interesting SciFi series ever.
Probably not enough of the mediocre typical TV viewing audience understood s1. One can only imagine that the forces of big-business media, which endlessly panders to the lowest common denominator, is the force which eroded what was initially an interesting concept.