I've seen both balance druids and elemental shaman played in naxx.
The druids can't keep up, period. I personally don't believe the utility of an innervate and a battlerez overcomes their considerable damage deficit. I know that their crit aura doesn't because I've run the numbers. At extremely high gear levels, elemental shaman can keep up with warlocks in several fights. Spell penetration gear and several bosses mysteriously lacking nature resistance after AQ makes it work.
Short of that, both specs will be weak in PVE, but both can still heal pretty effectively in those specs. Probably shaman more so than druid.
For PVP, elemental shaman are devastating. Burst damage, burst healing, windfury and tremor totem. You can do really well in PVP with very modest gear.
I can't say as much for moonkin, but that's just my experience as a player with over 15,000 hours in vanilla.