I always viewed it kinda like this. Value scale from 1-10 with 10 being the highest:
LT: 10
RT: 7
C: 5
RG: 4
LG: 3
The LT faces the fastest and best pass rushers and protects the QB's blind side, thus the premium. The RT is the stalwart against the 2nd best pass rusher and a bastion of the running game, very important position. Center is rated so highly because he makes the line calls and so often needs to switch which guard he is assisting or which guard is assisting him. Right Guard is generally a pulling guard because he doesn't do a lot of pass pro but is usually asked to reach the 2nd level, open holes run over his gaps, and win the battles in the trenches. In pass pro he's rarely facing a strong pass rusher and usually has help, so teams tend to want a RG who can run block very well and pull, work well in space, and be effective at the 2nd level. The LG sometimes pulls, but often helps the LT or C in pass pro, and most teams tend to run right more than they run left. When teams run left they often shift the formation to give them an advantage since LT's and LG's tend to be more pass pro oriented than run oriented (particularly LT's).
In the end, all 3 interior linemen have far lesser requirements for athleticism, so your pool of potentially capable players is vastly larger, probably 10 times the size of the pool of players who can play tackle. That's why so often a collegiate tackle will be adjusted to an NFL guard, usually because they have physical drawbacks like slow feet, short arms, etc.
If a kid can play tackle, they're going to play him there, if he cannot play tackle, they try him at guard or center, it's that simple.
EDIT:
Having now read the article, I'd have to say I disagree! He is concentrating almost exclusively on difficulty of pass protection, when about half (usually more than half) of the plays are running plays. Having a center like Dwight Stevenson was game-changing and if I had an elite center there's no way I'd try to convert him into a guard. I think this author is going on a limited set of questions given to linemen without really understanding how blocking affects the game. He rarely mentions the backs, he does talk about rolling protection and reaching the 2nd level, but he leaves the TE and FB out of the discussion. Most importantly, he really ignores the run game ... that's like trying to play with only 6 players ... just not the whole story.
While most of what this guy says about pass protection is correct, it's what he's not saying that really makes this article misleading. I'm of the mind that a half-truth ends up being more damaging than saying nothing at all, and that's exactly what this article is.
