My .02:
Enterprise failed (if a show lasting 4 seasons could be considered a failure...many Fox shows would
LOVE to have failed by that definition) because it tried to be too many things to too many people, and ended up pleasing virtually none of them.
In 1987, there was a drought of SF shows on TV. You had, what, "Beauty and the Beast" and ST:TNG? Maybe you could include "The Storyteller". So when TNG came on, we watched it with hungry desperation. I remember very, very clearly watching each episode and thinking "Well....
that could have been worse, I guess." We were very forgiving when we saw episodes that seemed to be BLATANT rip-offs of the TOS episodes....and of stories that were ungodly weak. Anyone remember the TNG episode where they rescue some 20th century people who were in cyrogenic suspension? [SHUDDER]
I don't think it coincidental that as Roddenberry fell away from the day-to-day work that the shows scripts and quality improved (even if the overused titles called "The X" (Such as The Call, The Hunted, the Price, The Defector...in the third season, they had a big string of 'The' shows). The show took some risks, and the stakes got more personal. Cliffhanger season enders got trite after a while, but "Best of Both Worlds" was a summertime stunner. Episodes like "Family", "Darmok" and quite possibly the best trek Ever, "The Inner Light" all made the series worth watching. That these shows hung solidly around Stewart probably isn't an accident, but episodes like "Brothers", where Spiner was given lots of room to shine, stand out as well.
DS9 started out strong out of the gate, but lost me by the third season. As someone else mentioned above, it didn't follow through on it's original premise. The intial conflict between the Bajorans and Cardassians was excellent, and whenever characters like Garak were given air-time, it was golden. However, many episodes felt very Trek formulaic, so when the many shifts started happening (the addition of Worf, the Dominion War) they felt forced to me, and I gradually lost interest. When I tried to come back, I'd found that the doctor had become a super-genius ("All those times we nearly died in the past? I was just pretending! Guys? ...Guys?") and they'd cycled a few actors. I just found it kind of forced. Some solid plots, but for various reasons, I dropped out. Biggest missed opportunity, to me? When Cisco and Quark are held prisoner by the Jem'hadar, Quark berates Cisco for the Federation's snide attitude about the Ferengi, hastening to point out how his race may be greedy and calculating...but they've never had genocidal wars, either. It was a great thread that could have been followed, making the Ferengi more than just comic relief, but it was never followed.
Voyager. Hmmm.....yeah, Voyager. Well, let's see. I gave it a chance, honest. B5 had raised the bar in terms of story, for me, so Voyager started out on a poor footing. I liked the continuation of the Maquis concept from TNG, and the initial concept had lots of promise. Implementation was AWFUL. What are the first two episodes following the premiere? The ship falls into a quantum singularity and a TIME episode. Compare that with the new BSG's first two episodes, "33" and "Water". Voyager's plots barely scratched the surface of their premise, while BSG's episodes give it a big, fat, wet kiss. Every time I returned to the Voyager well, I felt bad for doing so, as the show seemed to get worse each time. I WANTED to like Voyager...but it just kept making me dislike it. And don't even get me started on the pandering that was Jeri Ryan. I mean, really now...could you try and confirm non-fans worst images any more than that?
Enterprise had potential, but it just never clicked. It was boring, predictable and only had little snippets of what I had hoped for. I mean, they had holodeck technology shown by what, the third episode? It just felt forced, to me. The complete rewriting of canon was fine...if it were used to the show's advantage. It didn't feel like it was, however, and more felt like an excuse to re-use old concepts from the ground floor, rather than actually redefine them.
So what does Trek need? A rest is one thing that might work. Fresh talent and a new eye is certainly another. Trek gradually got away from the elements, story-wise, that made it so popular in the first place, IMHO. Being afraid or unable to change things over time except at season enders became a major stumbling block, for me. I don't need massive change every episode, but if I have to choke down on disbelief from the lack of continuity from one episode to another, why am I even watching? Trek needs to refocus, and remember that it's not the F/X that is making BSG popular, but the characters and the story.
Science Fiction doesn't have to be about big budgets....but about good stories, well told. Seven of Nine fighting in an intergalactic bloodsport against The Rock is not one of those.
