The smaller the team, the more shaman suffers. In a raid, shaman is great for healing. No CC makes it hard to justify a slot for elemental and enhancement (depending on your set up).
BG's, shaman do alright. The larger the bg, the better they do. I personally do well in AV, but that's a disaster of a BG at the moment so lol yeah. A shaman solo is a dead shaman at 70.
Arena's -.- I've only done 3v3 so far, and I hate it. You feel like you are in an uphill battle from the beginning. Everyone targets you and you are stun-locked/feared to death. 95% of the time I'm killed in the first 30s of the arena and just have to watch my team die. Kinda sucks tbh. I saw an article with the number of classes with 1800+ ratings. Shaman was low in 2v2 and 3v3 and above average in 5v5. When I say low... I think shaman was THE lowest class in 2v2 and second lowest in 3v3 lol. Priest and Warrior were even across all 3 arenas (and fairly high). Druid was the inverted Shaman doing better in smaller groups.
Enhancement is probably the least desirable spec at 70 because their burst damage just doesn't compare to other melee. Factor in that they can't stun and can't intercept or run fast... and they have a hard time actually staying ON their target. Almost every job can rather easily get away from them, and once that happens the shaman will never get close enough to damage that person again.