In 2008, the Steelers won a Super Bowl with what many consider the worst line to win a championship. Center Justin Hartwig was pancake blocked into the endzone and called for holding, resulting in an ill timed safety. In each playoff game of the 2010 run, interior pressure resulted the opposing defense scoring in some way.
Guard play is the team's biggest liability. This is especially infuriating considering the potential value of the position. Alan Faneca is a recent outstanding guard on the Steelers, and possibly the most respected offensive player of the 2005 team. While he played guard for the Steelers (98-07), Pittsburgh finished in the top 5 teams by rushing yards a total of five times. Since then the team placed in the bottom half of the league every season except 2010, when they managed 11th. Meanwhile the Jets signed Faneca and led the league in rushing in 2009.
"Rushing yards aren't everything," you might say. "The team is more pass-centric now." This is true, but consider the importance of guards in the passing game. The simple act of providing reliable pocket integrity is of huge importance to long developing plays and bombs where the QB must step into the throw for the pass to be both deep and accurate. The 80s Raiders and Redskins did not have Hall of Fame QBs or legendary defenses; the definitive player on those teams may have been the left guard (Gene Upshaw and Russ Grimm).
In today's NFL, elite guard play has translated into offensive success for the Saints, Packers, and Patriots. The Ravens have an excellent guard tandem, without whom Joe Flacco would be in no way capable of beating Pittsburgh's defense. Today's Steelers offense is inconsistent in big plays, unable to run reliably, and rarely fails to turn the ball over in big games. This is particularly bad when the team philosophy seems focus on "toxic" (turnovers and explosive plays) differential. OC Bruce Arians seems to favor play design that might be better suited for those 1980s Redskin squads. The results have been... mixed. Our team's offensive success has hinged upon Ben's and Mendenhall's abilities to make defenders miss behind the line of scrimmage.
There are other thin or aging positions (most notably ILB) where the Steelers might intelligently spend a high draft pick. However, an upgrade to guard would excite me the most. Not only is guard our biggest weakness, but the position also has the potential to define a championship offense.