When I was a young shaman, wielding my Vanir's Right Fist of Brutality with pride, I gemmed strength. Bold Crimson Spinels were the order of the expansion, because every point of strength was worth two points of attack power and scaled with Blessing of Kings. Agility gave us absolutely no attack power but did contribute to our melee critical chance. With no attack power gems in existence, Bold strength gems were placed in every available gem slot ever.
Wrath of the Lich King saw a change in our gemming strategy. Out went our plate-wearer style stat formulas; in came agility gear! One agility became equal to one attack power and a small amount of melee crit, while one strength gave us just one attack power. Unfortunately, we still didn't get nearly enough of a bonus from our agility. Attack power gems gave twice the attack power of an equivalent agility gem, and the extra crit wasn't enough to make up for an extra 20 attack power. Later on in the expansion, haste was such a brokenly good stat for us that it eclipsed even attack power. Agility spent a second expansion playing second string.
Now, Cataclysm is here, and it is finally agility's turn to shine. Much like the past two expansions, enhancement shaman gearing in Cataclysm is a very strange beast. This post is here to explain what each stat does for enhancement and where it's valued when you're gearing.