For me, I think the big difference/reason is that, like seigezunt alluded to, I could "feel" the vibrancy in the TOS characters.
The TNG ones, unfortunately, to me, even after seven years on TV and 3 movies (never cared enough to see NEM, either fortunately or unfortunately), are... nothing. Nada. Ciphers. Zilch. About as tasty as Wonder Bread.
Seriously. To me, none of them in particular stand out or "hook" a scene... Just IMO, of course.
Cheers,
-CM-