Part of the problem is that the jetpack or other flight become the default mode of movement. They're all too powerful. Or you can recharge them on the fly. I like Mekanism, because it's jetpack needs hydrogen generated by a fixed, power using structure. You can't recharge it in flight, or even just in the inventory. You have to haul around a gas tank and place it to recharge the jetpack.
Isn't that also true of the other jetpacks, at least until you reach higher resource manufacturing levels (enderium, etc
)? You still need to haul around some sort of battery (...Gas tank...It's still a block that you connected to other blocks and put resources in to make energy) and set down somewhere to recharge it when it's empty. Just saying. The mechanic isn't that different.
If you have no jetpack a horse becomes default (unless you're playing 1.9/10 with the new boats and end up in a very river-y landscape, in which case sometimes a boat is more useful and faster from point A to point B, even with all the river zigzags, than a horse that cannot swim).
If you can't find a saddle (pure LUCK, no skill involved, no building, no question of resources) then your feet become default. As I said, it's different strokes for different folks. You feel that jetpacks are "too easy" but the list of things that make the game easier is very long. I find automated mining and factory capability makes far, far more of an impact on the game than any travel mode does.
Oh, and leave us not forget mods like Mystcraft, where once you have linking books they become the default...You only have to go anywhere ONCE to set up a link.
As Azzanine pointed out, if something in the game isn't to your liking, you don't have to build it until later...if at all.
But I think we are delving into "What should Minecraft be?" territory instead of "What should this modpack be?" aren't we?