After a mixed reaction to the prequel trilogy, familiar faces are guaranteed to help sell the next Star Wars installment - to both die-hards and non-fans alike.
For fans, appearances by Han, Luke, and Leia (among other possible characters) will aid in washing away some of those (aforementioned) pesky prequel problems. Much of the current Star Wars franchise landscape has been based in the past - especially with the success of the (quality) Star Wars: The Clone Wars computer animated series. As a result, viewers will be ready for a jump into the future and a return to the original cast of characters that kicked-off the saga.
For non-fans, who may not be quite as connected to the ebb and flow of the franchise, Luke Skywalker, Princess Leia, and Han Solo are still cultural touchstones. Considering modern remakes of retro TV shows and films are all the rage these days, direct ties to the original Star Wars trilogy could re-energize less-impassioned viewers who might have been underwhelmed by the prequel trilogy (or the Episode 1 3D rerelease).