I would expect a shield to be +2 vs everything around you to represent the shields general protective ability. Then add +2 to every enemy that is in front of, or to the shield side of you. Then you should be able to add even more protecton with feats, talents, whatever. Not to go into all the different types of shields or ways they could be handled in game. I just hope shields get a complete reworking, and actually become usefull like they are in real life.