I don't think the mental aspect is a problem for Johnson -- in his first year playing he clearly got it. In fact, I think that in camp in '03 he had already shown that he understood the mental side -- which is why BP made him the starter almost immediately. Contrast this to Gurode -- after 4 years Andre still makes serious mental errors at Center. In fact, in one game last year (the 2nd NY game I believe), Gurode got snaps at C and BP called him out for making 3 mental errors in something like 11 plays.
A point I've brought up several times regarding Johnson's strength is that he had a serious knee surgery -- as part of his recovery, he likely was encouraged to lose a lot of weight to keep pressure off the knee. So, we've got a guy who already stuggles with size getting smaller as part of his rehab. When he was cleared to workout again, he likely had a long way to go just to get back to 290ish. This means that his first full fledged, non injured off-season was before the '05 year. I'd like to see what he can do with a second, non-injured year in the program.
Another point folks bring up about Johnson is that Bill said something to the extent of "he can't get any bigger" - I think folks interpreted this as meaning that he was NEVER going to be able to get bigger. However, if you listen to Bill's PCs you'll find that he generally does not talk in broad terms about player's futures. His comments likely meant that Al wasn't going to get any bigger during the season. That comment may have reflected a) Bill's frustration with Johnson's off-season progress, b) the fact that Bill doesn't want guys trying to bulk up during the season as this uses up too much of their energy for playing, and c) that Al was going to need to focus on his technique because he isn't going to overpower people.
My take is this. We have a center with all mental tools you want. He needs to put on 10-15 pounds if he can. But most importantly, he need to become a better technical player.
As I see it, Al's biggest area of improvement is technique -- and that's an area that you CAN expect a player to get better at.