I don't know if mind flay still has the mind seer effect at 20 (despite the spellbook saying level 26) but if it still does, the addition of
http://www.wowhead.com/spell=109142
as a lvl 15 talent can make shadow a huge threat in any flag room situations with the aoe pressure that shadow can pull off. The instant damage from shadow word pain on a target below 35% health triggers the 20% buff as well.
Shadow has a decent amount of resource management and things to keep track of which keeps things interesting. The damage is great right now too.
One thing to keep in mind though, playing shadow without the lvl 30 talent
http://www.wowhead.com/spell=64129 makes fighting any strong melee practically impossible with psychic scream on cd. I rolled a worgen for my 20 priest and it's a little more tolerable.