The gray spaces represent crew areas: cabins, crew and officers mess, corridors, crew recreation areas, offices, etc.
The main drawback to deck 1 is that the corridor doesn't run the entire length of the ship so you'll have to go to higher decks to get to some areas of the ship. No access to the aft stairs that are a quick way to get to Parrot Cay and Animator's Palatte. The midship elevators are sometimes busier than the forward and aft ones.
Another advantage/disadvantage depending on your point of view is that deck one is where the tender lobbies are located. That's where everyone gets on and off the ship in the ports.