I was reviewing one of my stories the other day and reflecting on the behaviour of some drugs (potions). It doesn't matter whether they are potions, spells, or other magical mechanisms, the effects would be the same. The first was a drug that caused a person to weigh less. Now, to be clear, the potion doesn't cause the person to lose mass, just weight. This has many applications, for performers, dancers, acrobats, swimmers (it would be better than a life-jacket), medical evac, climbers (rope and mountain) and recreational uses (nudge, nudge).
However, such effects, when presented on screen (TV or movie) always seem fake. I have realized why: landing speed and momentum. Certainly a person jumping from a fixed height (say off a 15 ft wall) would hit the ground at a lower speed. If we supposed that the spell reduced weight by 50% (which is substantial), then the impact speed would be reduced by 29%. Only 29%. The fall time would be increased by 41%, but that 29% is not much.
More importantly, if a person jumped and then landed on the same surface (like a figure skater, an gymnast, a dancer), the magic makes no difference to how hard they land. Certainly, they rise further (about double - exactly double if one ignores the offset strength required to just stand) and can perform more spins, flips etc in the "hang-time" (41% increase) but, ignoring air resistance which is negligible for 50% weight reduction, they will land at the same speed they took off. That means that if a dancer misses a landing from a leap, they will bruise just as badly with or without the drug.
The other effect is flight. This is a staple of many a fantasy story (or game). Now, in a novel or short story, I don't want to see all (or any of) the following answered. However, anyone who uses a flight spell will need to know the answers or suffer nasty consequences. How does it work? That is crucial. Is that propelling you against the air or against the ground? How long does it last? How stable is the flight at top speed? What about hitting birds insects, dust? How do you protect your eyes? Breathe? How fast (or soon) can the spell be recast? What about take-off and landing angles? Highest altitude? Altitude sickness? What about climb rates? Stable descent rates? Accidents? Running into trees?
Remember that even a 20 ft fall onto hard ground will kill most people most of the time (and that's not high enough to avoid running into trees). Does your hero have special toughness to deal with that? And I mean something magical because no Schwarzenegger-like toughness will save your hero from this kind of damage (nor will conventional armour). This rips muscles, tears tendons, breaks bones from the deceleration and distortion, not from the surface "hit". Sure, he's mentally tough, but flesh has a breaking point regardless of whether he is a book-worm or is Conan.
In a world that doesn't have trains, planes and cars, 15 mph is fast ! Not for us, but for them a galloping horse is the highest easily imagined speed and that tops out around 30 mhp. Most birds can fly faster than that, but most generally don't. Wind speeds over tall mountains can frequently reach 100 mph (and that's not storm conditions). Even on the flat ground, storms of any significant strength reach 20 mph or even 30 mph (and that doesn't include gusts). Any wind capable of lifting a flag is 10 mph or more. So what is the benefit of even a 30 mph flight spell?
Supposing 30 mph air-speed on a calm day for 10 minutes (that's a long and game-changing spell in most systems), that's only 5 miles.