I reckon its gerrard's inability to play the holding role in midfield that prevents him from becoming one of the best in the world.
at one point I thought he could be a world class box to box midfielder, but the defensive side of his game never really improved enough., a few years back,

Mascherano keeps his passing more simple, and as a result more effective, than gerrard too. gerrard looks for the defence splitting pass too much imo.