So what is it really about the Intrepid and Defiant classes? Why (on paper) does the USS Defiant pack nearly the same wallop as a Galaxy-class ship, for example?
I think I commented in the original "Starship Power Comparison" thread on why I thought this oddness might have happened, but to reiterate:
1. Where the Galaxy class has only 3 Mark XXV photon torpedo launchers, the Intrepid class has 5, and the Defiant class actually has 4 Q-II quantum torpedo launchers. (And quantum torpedoes are just crazy powerful.)
2. The Intrepid class, in my judgment, is ridiculously overloaded on direct-fire weapons. If it were any other class, there's no way it would be said in canon sources to have the same number of Type X phasers as a Galaxy class vessel: 13.
3. Voyager and Defiant are both more maneuverable -- Defiant considerably more so -- than a Galaxy-class ship. And Maneuverability factors into the Combat rating as noted in my original post above.
4. The Defiant class is on the books as a "warship," compared to the multi-role design of the Galaxy class. Given that the Defiant class is all about combat and carries the latest weapons systems (at least as of the 2370s), it's at least conceivable that its Combat rating might approach that of a Galaxy-class ship.
5. Here's the big one: Voyager and Defiant were "hero" ships -- they were the ships in which the stars of a Star Trek series traveled. Consequently, there was no power in the galaxy that could completely destroy those ships while Our Heroes were in them. This was also true of the Enterprise-D, but it was older. (And yes, I know the original Defiant was destroyed... but the stars of the series had -- just barely -- fled the burning ship in its escape pods before its destruction.)
To cover the nonsensical invulnerability of Voyager and Defiant with a veil of plausibility, Voyager and Defiant were given a completely silly number of weapons... and that punches up their Combat rating to nearly match that of the older and slower but much larger Galaxy class.
I'm not sure any of this should persuade you that it's OK for the Enterprise-D to be only slightly stronger in pure combat compared to Voyager and Defiant.
But if I've introduced even a little doubt, my mission is accomplished. :)