As a former OL when Johnson was OC at GSU, I will try to describe the various OL positions. I played all 5 OL positions there between 1984 and 1987, so I know a little about the differences.
If I had to answer the question, I'd probably say the center is the more challenging as 1) you gotta snap the ball and then execute the blocking assignment and 2) he sometimes blocks like a Guard and sometimes blocks like a Tackle.
I'll try to explain .....
The Guards must be good at:
1) base blocking
2) pulling (haven't seen a lot of pulling guards this year)
3) "scooping" - Scooping is a term meaning to seal off the backside on a running play. The linemen do not have an individual person to block ... they are blocking a zone. For instance, the backside guard will step flat (towards the center) and turn up field with the center. Anyone who attempts to enter the gap between him and the center (referred to as the backside A-Gap) is his responsibility. If no one attempts to enter his zone, he proceeds to level 2 (LB's) and, if no one is home, he proceeds to level 3 (Safety, Corners, or any other dummy standing around).
The Tackles must be good at:
1) Scooping (sealing off the backside B Gap) ... same as the Guard
2) Blocking the 2nd Level (and this can be difficulty). Most defenses are designed on the premise that the DL are responsible to defend a zone (e.g. - C-Gap responsibility), but also keep the OT off of the LB's. The DL over the Tackle is the first read on the option; thus, he is unblocked. He attempts to keep the OT from getting off the line clear to the 2nd level (similar to press coverage on WR's). Thus, OT's may not need to be quite as strong as guards (less base blocking), but they must be agile and have very good feet (e.g. - Weebles wobble, but they don't fall down). In fact, some of the best OT's in this offense (in my day) were oversized high school LB's. They were guys around 6'1 - 6'3", great feet, athletic, but only ran a 4.85 - 5.0 40-yard dash.
Guards must be bulls.
Tackles must have better feet and agility (maybe a little more gifted athletically?).
The Center ... well, he needs to be a little of both. Sometimes he base blocks and sometimes he is trying to get to the 2nd level with a big Nosetackle trying to keep him off of the LB's.
Pass Blocking???
Most pass plays in this offense are set up by the running game. Thus, pass protection skills will not be nearly as good as those teams who throw 30 - 40 times a game. Then again, this would be expected for a team with a run-first philosophy. The Tackle is probably the most difficult position on pass plays as they are "on an island". With no help or a free path to one side, agility and foot quickness are a premium. Then again, the same skills are needed by the Tackles for running plays (this works out good).
..... maybe this helps a little.