What are the most important statistics for a shaman?

Short answer: INT, STA, and SPI.

Long answer: it is really based on your build. If you wish to make a more melee based shaman, then strength, agility, and stamina are probably things you wish to maximize. Some shamans find a agility based build to be more to their taste. Given that we are an hybrid class with decent melee, there is no really wrong answer. And given that all statistic maximizing is done with equipment, you can keep all your bases covered with a varied selection of choice equipment stored in your vault.

Crit/Dodge chance calculation at 60: Shaman [AGI / 19.7]

Atk Power: (Character Level * 2) + (Strength * 2) - 20

Strength also increases the amount of damage you block with a shield.

A standard caster build emphasizes INT, STA, and SPI. Intellect adds 15 mana per point, and 1% chance to cast a crit spell every 100 INT. Stamina adds 10 HP per point. Spirit increases mana regen by 1 mana per tick (2 seconds) every 5 SPI, and 1 health per tick every 9 spirit, but for Shaman it's pointless due to our horrible returns from Spirit and no in-combat spirit regen talent.