Attribute Enhancement by Gears

Spell power:

With the new concept of Spell Power in WLK expansion, Priest becomes to the biggest winner. Never need two sets of gears anymore. Now Shadow Priest and Holy Priest can both strengthened by Spell Power. For Shadow Priest, Spell Power is always the most important attribute which can increase all kinds DOT and spell damages efficiently.

Hit Rating:

Hit Rating itself is an very important attributes. But for Shadow Priest while at outdoors, he only needs 4%-6% spell Hit Chance to make sure no miss on target. The talent already has 6% Hit Rating improvement. Therefore, Priest doesn't need to stack up Hit Rating points ignoring Instance and Raid.

Crit Chance:

Now Ming Flay and all DOT effects can do critical hit in WLK expansion. Crit Chance now becomes very important for Shadow Priest. Shadow Priest now needs to stack up some Crit Gear. Especially after choosing the talent points of more crit damage, high Crit Chance will improve Shadow Priest DPS outcome dramatically.

Spell Haste:

Spell Haste can decrease spell casting time. As Shadow Priest is using DOT as main spells, the effect of is no longer that important compared with Crit Chance and Spell Power.

Intellect:

Intellect is an important attribute which can increase spell power and regain certain Mana every 5 seconds.

Spirit:

This is also a good attribute for Priest

In conclusion, for Shadow Priest, the most import attributes are as follows: Spell Power/Crit Chance/Spell Haste. Basically no need to build up Hit Rating points.