For me a successful season is giving the fan base something to root for throughout the summer. While this will most likely insinuate an above .500 record, I think that the Phillies being competitive and within striking distance of the race should be their main goal.
If they carry the success they had towards the end of last year with young guys stepping up and filling their roles, they should be in most games. Mainly, towards the end of last year they were fun to watch. Secondarily, in accomplishing this, they would also likely be grooming their young players to reach their full potential.
If the Phillies can be somewhat competitive during the summer and simultaneously create a foundation for years to come, I will consider it a success.