If cast when the druid is not on an enemy's aggro table, or is somehow engaged with a mob, it is common for the treant to return to the caster without attacking anything, unless the mob was aggroed by having the treant cast directly onto it.
If the treant is the only one attacking a target for the entire duration of the fight and ends up killing it, the target will not yield any Experience or Honor. This is especially important to keep in mind when helping lower-level players complete an instance - if your treant kills a boss by itself, no one will get quest credit for it.
Treant's health and damage scales with the druid it belongs to.
Casting Faerie Fire on a mob that your treant is attacking will increase its damage output.
The treant can provide a very substantial amount of DPS in a boss fight, but it is important to time the cast so that the treant is out of danger of being annihilated by AoE. It is also important that you cast it at the back of the boss, so that it doesn't cause the boss to parry, and so it doesn't die due to cleaves.
Against priests of all specs, it is a good idea to summon the treant soon after they cast Psychic Scream so that they cannot instantly nullify your treant.
Patch 6.0.2 (October 14, 2014): Force of Nature Treant's healing now benefits from Mastery.
Patch 5.4.0 (10-Sep-2013) Feral: Treant summoned by the Feral version of this talent now deals less melee damage, but will cast Rake and Entangling Roots on their target instead of Bash. Guardian: Guardian Druids now gain Vengeance when their Treant takes damage.
Patch 5.3.0 (21-May-2013): Will now chain cast spells with no delay between casts, allowing treants to cast their spells more frequently in the same amount of time.; Force of Nature is no longer on global cooldown and summons a single Treant. The Treant no longer has a control bar, immediately uses its special abilities on the Druid’s current target, and accumulates 1 charge every 20 seconds up to a maximum of 3 charges.; Restoration version of the Treant now casts Swiftmend on the Druid's target when summoned. This version of Swiftmend does not require or consume a heal-over-time effect on the target.
Patch 5.2.0 (5-Mar-2013): The Treants summoned by Force of Nature now deal more damage and healing, and the Force of Nature tooltip will report the capabilities of these summoned pets.
Patch 3.3.0 (08-Dec-2009): Health on the treants has been increased.
/Patch 3.0.2 (14-Oct-2008): Cooldown reduced to 2 minutes, down from 3.
Patch 2.3.0 (13-Nov-2007): This ability will no longer automatically break stealth on victims in its casting area. Nearby stealthed characters may still be attacked if they are too close to the summoned Treants.
Patch 2.2.0 (25-Sep-2007): Using this spell in a neutral town will no longer cause its victim to be attacked by town guards. It also will no longer cause nearby party members to lose stealth.
Patch 2.1.2 (2007-06-19): This spell will no longer causes players it is cast on to enter combat with nearby creatures. All of the treants summoned will now attack a neutral creature they are summoned on.
Patch 2.1.0 (22-May-2007): The Treants summoned by this spell will now correctly attack your target even if it is neutral or non-aggressive. These pets will now come into the game with full health, including that gained from a percentage of master’s stamina.