Even though it is 'instant cast', it still takes a half-second or second to cast depending on server response time. Remember this if you try jumping and then casting.
If used and before taking a running jump from a great height, the reduced fall speed will increase the horizontal distance traveled and move a player's character away from the cliff while descending. This effect can be minimized by casting the spell mid-fall.
[Levitate] is a similar spell used by priests. A notable difference is the fact that levitate breaks on damage, allows water walking, cannot be used while mounted, and allows the player to float in the air at a height roughly equal to a standard jump.
Having a mage cast this upon you is one of the quickest ways for new players to safely obtain the achievement [Going Down?].
Tips and tactics
Even with a duration of 30 seconds, you rarely need to cast it again while in midair after jumping or falling since 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. It can also be used to quickly transport players from the Lumber Mill to the Blacksmith even in other situations.
It is often used in Eye of the Storm to help other players quickly and safely reach the flag at the center of the battleground at the beginning of the game.
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 cast Slow Fall all the way to Darkshore (although multiple Slow Falls will need to be cast).
Ironically, when this spell is cast, the falling physics in the game become more realistic than usual with regard to Newtonian physics, since without the spell, terminal velocity is far too high.
Slow Fall doesn't reduce your horizontal velocity, so using it too early may send you too far. If this is an issue, using "Blink" will remove all forward momentum and instantly place you on the closest landable surface below you. While this can allow some precision, it can also condemn the player if the target is undershot.
Enemy players with flying mounts can simply follow you as you fall, preventing you from escaping combat; again, using [Blink] can help.
Patch 5.0.4 (2012-08-28): No longer requires Light Feather.
Patch 3.0.8 (2009-01-20): Slow Fall is now castable on others.