Not much, did you think about downsides tho?
Right now one directional key press = 1 packet, with free movement it will move by 1 unit (to make the movement smooth), lets say 1 tile = 32 units. That's 32 packets just to move through a tile. Then there are animations that will look really clunky with such movement. Also onStepIn/Out will need rework because if a tile is on pos 128, 128, 7, then to check onStep you would have to check if player is within range (128, 128, 7 to 128 + 32, 128 + 32, 7).