I know there are alot of OT fans who are excited to see the main cast as the center but I think its a mistake.
The 1977 original was essentially a sci fi film for teenagers and young adults. The kid, the youngish rogue and the smart princess taking on authority, making jokes,and saving the universe. There was that cool villian Vader and the mentor Obi Wan was neat but people walking out of that film really loved and identified with Luke and Han and their journey as younger growing characters.
As much as people here dislike the prequels, it tried to keep that tradition with the main three characters.. It was just overtly more serious, amongst other things.
To have an episode of star wars to be about three old people running around the universe just doesn't work, at least in theory. Old cliffhanger serials that star wars was based on kept the cast usually pretty young. Personally I was fine with seeing some young new characters and Luke as a mentor. I think thats what Arndt wanted and it kept with the concept of what these movies are about. Escapism for kids/teens that adults wont detest.
From the latest postings at TFN, it sounded like Lucas wanted the young main characters but Abrahms and Kennedy said no. Whatever you think of Lucas, he created this entire concept. Kathleen Kennedy has worked with Spielberg for the last twenty years to create forgetable audience pictures with very few good exceptions. And Abrams if he really thinks that Star Wars is only about 3 OT characters including one who was barely used in Jedi (Han) then hes mistaken.