While in this form, druids cannot cast healing spells.
When in Moonkin Form druids are flagged as a humanoid and are immune to beast-type magic, like a Druid's Hibernate, or Hunter abilities designed to affect beasts, but they are also immune to polymorph.
Moonkin is one of only two shapeshift forms in which a druid can deal weapon damage. (The other is Tree of Life form.) In feral forms, damage dealt is feral, not weapon damage.
Druids cannot ride certain mounts while in Moonkin Form. They will also need to remove Moonkin Form in order to use a flight path; unlike the other forms, talking to a Flight Master NPC will not force the Druid out of Moonkin Form. There are a few places where change of appearance can allow you to ride mounts in Moonkin Form, such as places were you get the fel orc costume, or vehicles in Ulduar. Shifting into Moonkin Form will not change your appearance, just give you the bonuses to Armor and Crit,and it is possible to mount. Leaving such an area will not dismount you but you will gain your moonkin appearance, making you a mounted moonkin. This same trick can be done in Wetlands with the Cursed Sailors.
Players have given the moonkin form many nicknames. These are normally short and end with -kin; some of them are: Boomkin, a moonkin druid with a high damage output; Oomkin, a moonkin with poor mana regen; Doomkin, a good PVP moonkin (rarely used); Noob/newbkin, a bad/inexperienced moonkin druid; and Panzerkin, a tanking moonkin. Sometimes these nicknames are used to describe a particular druid, but most of the time players use them as a general term. Many moonkin druids find some of these nicknames insulting.
Moonkin tanks ("Panzerkins") can be a strong alternative to non-tank specs through decent damage mitigation and high DPS (versus main tank specs). Moonkin tanking is rare for many reasons, but Moonkin have successfully tanked Karazhan, Gruul's Lair and most 5-man heroics. Gearing is problematic; PvE rewards do not generally support Moonkin tanking, but the Arena Rewards for druids provide some of the balance necessary. Keep in mind that moonkin armor is equivalent to plate, but will still be lower than most warriors, feral druids, and paladins because of the lack of armor-increasing talents and abilities.
Moonkin have several abilities that are key to surviving in PvE. Among them are [Cyclone], [Entangling Roots], and [Barkskin]. Cyclone and Entangling Roots gives the Moonkin CC capability which is essential for surviving accidental overpulls or aggro.
Barkskin is useful for mitigating damage and also allows the druid to shift back to caster form to cast healing spells. Barkskin helps to reduce armor lost from shifting out of Moonkin Form.
Moonkin may also consider using the [Typhoon] talent which knocks targets back and dazes them for a short period of time.
Balance druids also have [Starfall], which is capable of AOE damage to enemies in a very wide area dealing high damage.
The key to PvP fights in maneuverability. The Lunar phase of [Eclipse] pumps out more DPS, so the druid will want to be in that whenever possible. While moving, instant cast spells are a must, especially [Moonfire]/[Sunfire], as use of them will proc [Lunar Shower] (level 82 ability). Plating [Wild Mushroom]s while on the run is smart, as is timing their detonation.
Obviously if the druid gets a chance to stand still they should use their high damage nuke abilities. Remember that [Solar Beam] can also be used against enemy casters.
Patch 4.0.6 (8-Feb-2011): Entering or leaving a shapeshift no longer cancels root effects. It continues to cancel movement slowing effects.; Moonkin Form now grants 15% damage reduction instead of increased armor. In addition, shapeshifting in or out of this form now breaks roots. Note this is not stated in the tooltip.; Druid forms now have an Archaeology Survey animation.; Druids in Moonkin Form should now correctly be able to mount.
Patch 3.2.2 (2009-09-22): This form now also reduces the damage the druid takes while stunned by 15%.
Patch 3.0.3 (04-Nov-2008): Only single target spell critical strikes can trigger the mana gain from this talent.
/Patch 3.0.2 (14-Oct-2008): No longer increases your attack power or causes you to regenerate mana off melee attacks, but now has a chance to cause you to instantly regenerate 2% of your total mana every time you critically hit with a spell.