Poor execution of concepts, storylines and characters is what weakened Trek, IMO.
VOY was a really good idea, a lone starship, braving the unknown to get home, making contact with more aliens and seeing more that any other crew in the history of Starfleet. But instead it just seemed to be a walk in the park which ended with them firing on every species they came across. There was nothing made of the need for resources, the loss of even the lowliest redshirt (in a situation where no crew replacements will be coming every single person is needed), no decimated morale from being so far from home and family. It all just seemed too easy, I mean look at how many times they beat/outsmarted the Borg (the Borg for heaven sake!).
Characters not being written in a consistent manner, some who were so annoying you just wanted to kick them in the crotch, and others who were so dull it would have been more fun watching plants grow in the hydroponics bay.
Yes there were some good episodes and character moments, but they are the silver-lining on some very large, dark and thundery clouds.
As for ENT, the prequel idea had merit. Go back and see how things began, but the continuity errors were just too many to let them get away with. As too was their reliance on already overused species (Klingons) and others who were supposed to have been met for the first time centuries later (Borg and Ferengi). Instead of looking at the birth of the Federation and the difficulties it would have faced in its early years, instead we go further back to mankinds first deep space mission, on a ship that looked far too advanced.
Add to that a Captain who was so inept I wouldn't have trusted him as a conductor on a train, an obviously token black guy, and the very-English English guy.
Again there were some nice storys and character moments (Trip and T'Pol saved the show in my eyes), and a season-long story arc was a nice touch (though DS9 had already done something similar with the Dominion War). But to me it was the wrong setting, the wrong ship and definately to wrong Captain.
What could save Trek?
Develop more rounded, better thought out and compelling characters, people who we actually care about or love to hate. Map out how characters will grow and evolve.
Don't rehash old storylines from previous series, or dredge up aliens who have been done to death (its a big galaxy out there). Look at telling interestingm thought-provoking stories that examine the human condition and current events in a sci-fi context, this will mean some dark plots now and then but so is life--the Trek optimism can be found by how the characters deal with these situations and grow.
Move away from the stand alone episodes, have longer arcs, an underlying backgroud plot, or storylines that keep cropping up.
Take risks. Don't play it so safe. Try something new and be daring. If something fails or doesn't work, learn the lessons and move onto something else. Put the characters through hell at times so we can watch as they rise to the challenge (or not) and succeed (or fail), and what that means for them afterwards.
Again, all this is just my own rambling opinions and thoughts, hopes and dreams.
