Ask any rookie about the most difficult part of coming into the NFL, and they will tell you it's mostly mental adjustments. Getting a feel for the increased speed of the game, learning the new playbooks and new ways of doing things, trying to make sure you don't get left behind and showing that you have something worth investing in.
This is especially true for quarterbacks entering the league. They have to memorize a brand new offense and practice until executing it becomes second nature, and they have to read defenses and make the proper adjustments while still dealing with the other hindrances slowing down rookies at other positions.