The Star Wars franchise doesn't have a flawless track-record when it comes to introducing new characters. While the prequels are notorious for a questionable roster of fresh faces, their were also plenty of controversial additions long before Episode 1: most notably the Ewoks.
However, assuming that returning characters in Episode 7 are a success (see Pro #1), they will undoubtedly create added pressure for the series going forward. In order for new trilogy characters to be worthwhile additions, they'll have to stand toe-to-toe with some of the most iconic and beloved characters ever seen onscreen - while also establishing their own stories and dramatic setups for future payoff. If new heroes are significantly less interesting than seeing old veterans reunite, Episode 7 will have missed the most important mark: to establish a new batch of fascinating and capable Star Wars adventurers.
An intelligent script and smart casting will be instrumental in the success of the new characters but, considering how eager fans are for more Han, Luke, and Leia, the filmmakers have a tricky balance ahead of them.