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 a running jump is taken from a great height, the reduced fall speed will increase the horizontal distance traveled and move you away from the cliff. This effect can be minimized by casting the spell mid-fall.
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 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.