Slow Fall is a utility spell which slows target's fall speed, allowing leaps off of great heights without taking any fall damage. Slow Fall requires a single reagent, [Light Feather], which is consumed upon casting the spell. As of Wrath of the Lich King there is a glyph that removes the need for a reagent, making this a very useful spell that costs almost no mana.
Even though it is 'instant cast', it still takes a half-second - second to cast depending on server response time. Remember this if you try jumping and then casting.
If used and then take a running jump from a great height, the reduced fall speed will increase the horizontal distance travelled and moving you away from the cliff. This effect can be minimized by casting the spell mid-fall.
Levitate is a similar spell Priests have which uses the same reagent. A notable difference is the fact that levitate breaks on damage, allows water walking, cannot be used while mounted, and raises the player in the air equal to a jump.
Tips and Tactics
Even with a duration of 30 seconds you rarely need to cast it again in mid air after jumping or falling as most falls are much shorter than that.
Slow Fall is a wonderful way to escape from other players by running off the nearest cliff. This strategy is particularly effective when escaping ganks on PvP servers.
An especially sneaky tactic used in Arathi Basin is to stay at the Lumber Mill and use Slow Fall to escape off the cliff if the mage is overwhelmed by other players. A sneak-attack can also be performed by safely jumping down the cliff to the Mine.
The extended horizontal distance can be used to reach places that normally a player would be unable to get to, especially if the mage casts the spell, mounts, and then takes a running jump off of a cliff. This may not be particularly practical, but it can lead to some interesting locations. With an epic mount, a player can jump off the southern half of Teldrassil and slowfall all the way to Darkshore (although multiple slowfalls will need to be cast).
Ironically, when this spell is cast the falling physics in the game become more realistic with regard to Newtonian physics than usual, as an unmodified fall in the game causes the player's velocity perpendicular to the pull of gravity to approach a limit and taper off, which would not happen in real life nearly as quickly assuming normal air-resistance at Earth sea-level. However, a measure of control is offered through the "Blink" spell. If a Slow Fall is carrying you too far, using "Blink" will remove all forward momentum and leave you floating straight down. While this can allow some precision, it can also condemn the player if the target is undershot.