There's no jet packs in Halo 4, there are thruster packs. I believe 343 has said that Thrusters provide a momentary speed boost in a given direction and no people have been seen "jet packing" in videos. There have been plenty of people with thruster packs on who are mistaken for people with Jet packs, but 343 has said they arent in and there has been no footage to the contrary. Just a word to the wise.
With regards to the OP, the only maps Jet pack really bothers me on is Asylum. Other than that, I could care less. Allot of people said Countdown as a problem map, but I've never had problems with jet packers on that. If anything, Jet Packing has routinely seemed to me to be a great way to get yourself killed except on those few maps which are structured in such a way as to make them usefull, and other than Asylum I can only think of one (its a forge world map that I can't remember the name of). With regards to map flow, Sweaty Bagels is only partly right as nearly as I can tell. Aside from a few very definite movements that jet packs don't really stop, Map flow emerges organically over a long period of time, it cannot simply be designed into a map. What map flow is designed, with regards to maps in Reach, remains regardless of Jetpack. The greatest kill area on Countdown is still the top of the lifts.