Voyager would have been THE opportunity to combine the serial storytelling of DS9 with the alien world of the week aspect of TNG. And put some dirt on the shininess.
I've been watching re-runs of VOY occasionally, and I actually like it. The characters are fine, I like Tuvok and the Doctor. Heck, even Neelix is entertaining most of the time. The problem is it could have been a lot more.
For one thing, Voyager shouldn't have been in that great shape for the entire show. There should have been problems, shortages, - heck - DIRTY uniforms would have even been something for a start. The relationships between the crew members where too "clean" as well, just like the TNG crew. They are 70 years away from home, in an EXTRAORDINAY situation, how about bending Starfleet protocols over the course of the show? They should have become a family, even more than the DS9 crew. But they stayed at that professional distance just like the TNG crew. The entire show is just too clean and sterile.
As a result, the show doesn't evolve at all. The first episode is like the last episode. Harry Kim staying an Ensign for the entire run of the show is the clearest sign of that.
Another aspect is the entire crew. I don't know how many there are supposed to be, 150 perhaps?
There are episodes where they are attacked and then Tuvok says "Engineering reports three dead, fifteen wounded.", and Janeway, or everyone else, just shrugs it off. That's not believable in any way.
They should have made a list of those 150 crew members right from the beginning, keep it consistent, and over the course of the show give each of those crew members a moment to shine, and then keep them in background scenes. Creating a real feel for the family aboard the ship. The bond amongst ALL crew members would grow so strong in such a scenario. And when one of them dies, its a big tragic event, not just something you can shrug off.
The Maquis / Starfleet plot should have been fleshed out more. It should have resulted in a truly unique Star Trek crew, even more unique than the DS9 crew, but in the end it was just like TNG.
And eventually, they relied on the Borg way, way, way too much.
And they didn't resolve the Caretaker storyline like it was promised in the first episode and occasionally hinted at during the seasons. In my opinion, the entire story arc should have come to a climax with Voyager finding another Caretaker who gets them back home.