Rejuvenation is a Heal over Time spell for druids, and is one of the earliest spells to become available to the class. The spell is not very mana efficient when first learned, but it scales very well with talents and spell power, and at higher levels it is a highly efficient healing spell.
[Nature's Splendor] increases the duration by 3 seconds, adding one additional tick at full strength. This results in a significant increase in the total healing done by the spell.
Subtlety reduces the threat caused by the spell by 30% and reduces the chance that it will be dispelled by 30%.
[Empowered Rejuvenation] increases the benefit the spell gains from spell power by 20%. Note that this is a 20% increase to the amount gained; it does not mean that Rejuvenation gains an additional 20% of your spell power.
The real strength of Rejuvenation is how well it scales with spell power, not the base spell itself. Until you start getting gear from Outland with significant amounts of spell power, Rejuvenation will do less healing per mana than both [Regrowth] and [Healing Touch] even fully talented.
How much benefit Rejuvenation gains from your spell power depends on a number of factors, including talents, the presence of [Improved Devotion] or Tree of Life Aura with the stronger effect being counted and the rank of the spell.
The final amount of healing over the base duration may be computed as following:
Example: With maximum benefits acquired a rank 15 Rejuvenation spell cast at level 80 may heal for 3563.43 + 396.4% of your healing spellpower as displayed in your character's paperdoll. A rank 10 Rejuvenation cast at level 80 however would incur a penalty[1] of 75% to its spellpower coefficient, thus it will only heal for 830.24 + 297.3% of your spellpower in this case.
Patch changes
Patch 4.0.1 (2010-10-12): Is available at level 8.
Patch 3.3.0 (2009-12-08): The base duration on all ranks of this spell is now 15 seconds. Previously ranks 1-14 had a 12 second duration.
Patch 3.2.2 (2009-09-22): (Source) The coefficient is 0.376 for both ranks, and in fact all ranks.