MLB GM rates the bullpens for the 2008 season and the Marlins finished 29th.
I don't believe this ranking for one second. Sure, it could happen since the bullpen had to be overused last season when most of the starters had trouble getting through five innings consistently. But it is doubtful that they will suffer a major setback, especially since it wasn't the same two or three guys pitching every night.
The Marlins bullpen was one of the bright spots of last year's team and if they remain healthy, could be again. (Aside: I'm factoring out the two disasters, Julio and Benitez.)
As for Gregg, it isn't necessarily true he will be a one hit wonder. In 2005 when Mota became injured, the closer job fell to Todd Jones. Up to that point, Jones had been ineffective in middle relief and everyone at FishStripes moaned at the announcement, me included. Jones ended up being a very good closer, albeit in that cardiac style of his, and about the only pitcher out of the bullpen who could be trusted. He later went on to sign with Detroit, as most former Marlins have, and is still their closer to this day.
You never know what will click with a player at a certain point in their career. It could easily be the case that Gregg has found his role and he will continue to do it well. And should he not be able to, the Marlins have other players who can close out the games.
Sure, they won't remind anyone of Treavor Hoffman in his prime, but they won't remind anyone of Jorge Julio either.