The original game had jetpacks and fake-skiing everywhere. That was back when players realized that tapping "jump" fast enough would cause you to coast down slopes at high speed - in future games skiing was made into an official feature. Jetpacks have a cooldown (or they always did) that let you fly a certain length of time and then it would overheat/discharge and you'd need to let it redo. For the "scout" class, you could feasibly taking a running jump off a tower and fly away, expending all your energy but gaining enough during freefall to cushion your landing and avoid splattering. Meanwhile assault classes would usually need to expend all of their jetpack to move up a medium hill (small exaggeration, but just a small one). It wasn't a case of everyone flying around all the time, you had to be tactical with your thrusts, and mis-thrusting could see you walking awkwardly or falling to your dead. The spinfusor was a prime weapon to use against jetpacks because of the high blast radius. It became rather easy to shoot where someone was going to land, and a skill to coordinate the strike to the landing and get an insta-kill. Meanwhile sniper lasers and chainguns did a fair job of getting rid of people who were in flight.
It balanced itself fairly well. Flying was very important, but not overpowered. Instead of flying it was more "bunny hopping" and that left you vulnerably plenty. Not to mention vehicles did a great job of chasing you down, and the missile-armed scout ships were faster than some missiles and capable on ramming attacks that would easily destroy bunkered turrets, not to mention let you ram someone who was running if you could pull off the strike. So much fun~