I believe that much of the success of the TOS movies is due to the participation of Nicholas Meyer.
I couldn't agree more. Nicholas Meyer knew how to direct a movie, and how to get really good acting out of Shatner. TWOK and TUC are my two favorite Trek films hands down. His darker, hard-edged militaristic approach really gave both those films a more dramatic look about them, which I believe TSFS was lacking, although I love that film too. The sets look a lot cheaper in the films he didn't direct.
He also helped write IV, which is also considered one of the best (although I'm not too fond of it).
The TNG films all suffered from a tired franchise, IMO, and because they didn't have the ensemble cast interplay that the TOS films had (for example, how all the TNG films were very Picard/Data centric)