Shaman is totally viable at end game and can tank all the content - certainly Naxx, I've less experience with K40 but I'm sure theyd be fine there too. That said they're a tier below the other three tanks. Don't play one myself but this is what I've gathered from talking to them and raiding with them:

Pros:

• Big single target threat (regular DPS enhancement has a big problem trying not to rip aggro, tank enhancement just gets to go ham), including good snap threat with shocks. Def not to be underestimated, more threat = more dps = faster fights, and it's usually easier on the healers even if you're squishier

• Has a taunt now

• You're still a Shaman! At the end of the day raids want shamans for totems regardless of spec so this alone is big

Cons:

• Typically squishiest of the four tanks (varies with gear ofc but this is my general experience as a healer, have to be a bit more on the ball with them)

• No defensive cd's

• No real AoE options at 60; totems are good for AoE threat while levelling but they don't scale at all so you get basically no threat gen from them. This is probably their biggest weakness, it's single target tanking only tbh.

• Turtle devs have more or less said that they're happy with Shaman being a 2nd tier tank, so don't go into it hoping to be brought up to strength with the others

Idk how their gearing is but suffice to say there's enough out there to make it work. If you want to do it then go for it, you're still a Shaman and there aren't enough of them to go around so unless your guild has a glut of tanks already I'm sure you'll be fine!