As I may have said before in the TNG thread, those TOS characters were so effective because the writers weren't afraid to highlight their very human character flaws. Spock was arrogant and condescending (and self-hating, to some extent), McCoy was grumpy and stubborn, possibly slightly bigoted but in an amusing and pragmatic way. Those two really disliked each other in the TV show, that's been my observation.
Kirk was a borderline sexual degenerate and, when he made one of his moralistic speeches, a complete hypocrite as well. But these flaws made these 3 characters to relatable to the audience, who could recognize traits they could see in themselves.
Wheres in the post TOS universe, for some reason Roddz wanted the characters (the human ones at least) to never have any personal flaws. So you wound up with Bev Crusher, Laforge, Wesley Crusher, Troi, Ezri Dax, Harry Kim, Paris, Chakotay, Belanna Torres (the two most boring terrorists in history, apparently), Mayweather, Tucker, Sato and Reed. All those character you could switch their personalities and it would take you an hour before you'd notice the difference.
I'd almost include Riker on that list too, but he was such an amazingly effective first officer (the best XO in all of Star Trek IMO) that I give him a pass.
