I definitely think you will be able to return to old places otherwise bungie wouldn't be able to keep having people on the server. They would get bored. I think once you get some loot from a place, it should either not be the same loot, or there should be no loot at all. By the time you decide to return to a place, you'll be a much higher level and if you get the same loot, it won't be anything special. I for one will constantly be returning to the exclusion zone on mars that the Cabal control. That place looks like a fun place to fight!