Joe Thomas is your classic positional blocker. He uses his quick feet to position himself in a way to wall off the defender. What makes Thomas so good is his quickness off the snap. If a positional run blocker is slow off the snap they will most likely not be able to gain a positional advantage over their opponent.
Levi Brown is also a positional run blocker despite his strength. Brown is very good at positioning himself and using his strength to turn the defender away from the play. He isn't that aggressive straight ahead mauler that some people are trying to make him out to be. The major problem with Brown is his pass blocking. I think he will struggle against the speed of the NFL DEs. Brown tends to plant his feet when he engages the DE. Once his feet stop moving NFL DEs will easily beat him with a couple different moves.