He doesn't speak! That's a hugely influential thing in an RPG whereby story is paramount in my opinion. That immediately limits the game in terms of the way the main character interacts (or doesn't...
) and relates with the plot and other characters. Again, that's the style and will have been done for a reason, but it doesn't change the fact that this is the case.
What I'm really getting at though is how the dialogue gets you to know the characters, and the focus in game on this aside from the direction the plot is moving towards. Even in regards to the plot, though...take the example of when Crono dies. The main character dies and this is obviously pretty significant, but there are only about 4 lines dedicated to this during and after! This is a very clear way of summing up the difference. Compare this with when Aeris dies in 7 and the emotion that is brought about during the dialogue between Cloud and Jenova. It is in a completely different league. It's something that really became apparent with the stuff between Lucca and Robo after you save Lucca's mum - it makes you realise that sort of thing is very rare, but it's that sort of thing in my opinion that adds the most depth to characters.
I'm not trying to be critical because this is more a reflection of the games at the time. FF6 like we both say is better for this but at the same time I think the difference with 7 and beyond is quite clear. The character development and dialogue in tandem and aside from the plot is far superior for me.