Okay this is going to sound really stupid, but what defines an MLG map? I'm pretty sure you've got to make realistic structures, but what else?
He means that pretty much your structures look architecturally sound. Lots of arches, columns, and support. The only exception I can think of is part of Amplified where there is a floating platform that really just had to stay floating. Either way though, having structural elements in a map can greatly contribute to its aesthetics and its gameplay value by cutting off and dictating critical Lines-of-Sight.