"Shaman should focus on raising their Willpower, Intelligence, Wounds, and Toughness most of all. Our two MAIN stats however are Willpower and Intelligence. Which should you focus on more? Right now that's tough to say. You're obviously a healer but you can deal damage as well. Think about which mastery you're focusing on. If you want to be more offensive then Intelligence is a natural fit. However, I strongly believe that all Shaman should focus on willpower as their main stat. Right now we have a hard enough time keeping a group healed up and if you skimp on the Willpower it's just going to be that much tougher."
Maybe its because they get a tactic which increases all damage by 25% but reducing healing by 20%.
There's a combat log parser available which may be a useful aid in analysing stats change effect on damage etc during open beta and beyond.