NOTE: System still in development and subject to change/tweaks.
General concept: This doesn't work like your typical achievement system in that it's integral to the game and character advancement.
In a nutshell, whatever level and tier you are, you will be able to bring up the Achievement system. This will show you, say, 20 tasks you can do, and you must do, say, 10 of them to advance to the next level or tier. One part of that will be a certain amount of exp gained, but that's not enough to level in Pantheon. You need to do the 10 out of 20 (these are just guesstimates) tasks, including the exp, in order to make it to the next level or tier.
So you have a choice, but you still need to finish a certain number of tasks (could be a quest, could be kill a certain mob, could be explore a certain area, could be a lot of things) to advance your character.
Here's a great post that explains it perhaps better than me:
I understand what you are saying mate but I really think that you are overthinking it, from that twitter conversation, reading your post, reading Brad's reply and the old Kickstarter info, no where did I see anything that said I was going to be forced to quest and explore whether I liked it or not.
My understanding is that we are free to explore the world at our own pace, whenever we like but each time we progress high enough to be able to "level" or move onto the "next tier" (which will be slow progression) we will then have to seek out a quest and prove ourselves to accomplish this. It doesn't sound like your average "ding you are level 20" it sounds like we will hit a cap of some sort and to progress, we will need to then complete a series of quests/tasks/challenges to be granted the new level/tier and I actually think that's a very cool way of handling the leveling system.
That will not stop you from exploring the world though or take away your freedom or force you down strict paths, it would only be when you need to progress and prove yourself worthy of that challenge to gain a new level or tier.
I wouldn't actually mind to have this tied to the leveling at large soto speak or something similar. Maybe that was what you were aiming for, but it would be... more