Kirk, Spock and McCoy are heros of the old school, the very foundation of adventure/drama.
In the three, ToS was able to play accross a wide emotional and intelectual spectrum, and touch something that fans still feel 4 decades later.
Watch those old episodes, and you really do feel space is vast, and new things can always be found, yet no matter the challenge, the three will find a way to overcome the problem, through ledership, logic and common sense, the three qualities of Kirk, Spock and McCoy.
This is the missing element, TNG and DS 9 both tapped into the hero base, you really carred about Data and Worf and O'Brian and Sisko and all the other characters.
I never felt that for Voyager, I didn't care about them, and I don't really about Enterprise, and to me, that is the problem.