I can understand why some people never got to 50 when there was nothing to really do after that at the time.
DFB and DIB are definitely the best starter trials. You should run SBB once a day for the event enhancement at least. Run all the basic TF's needed for the Task Force Commander accolade, and others can be helpful like Moonfire to help get those vampire and werewolf badges need for the Atlas Medallion accolade . Once you get to 35 get your Member of Vanguard badge so you can start getting vanguard merits, which can be converted to merits rewards at 30-1. With the vanguard badge look to run Mother Ship Raids, good exp and merits. Also after 35 one of the best ways to exp is running ITF's. At 45 you can join Hami raids. With some mission groups here and there you should be able to hit 50 pretty quick.
Having leveled a number of 50's without PL'ing (nothing wrong with that, I like the ride) I think one of the things that helps keep you from straying with alt-itis is having a build you really like. With so many options it can be hard to find the AT and power sets that trips the trigger, but once you do it can be huge. For me it was a gravity - kinetic controller.
