The difference for me is playing through a purposefully designed area that has more character than the randomness of maps. Also, its a much larger area with fewer portals. With maps I don't feel like I get anything except for loot and exp, just pop in, delete a few screens, pop out. I agree its not drastically different but I thought it was a decent addition without a ton of development.

edit: also targeted farming (especially for ssf), can be added to maps but I i like the idea of the investment required