Just for a sake of argument: Let we say that Act 1 is 1-6 level, Act 2 is 6-10 level and Act 3 is 10-12 level.
My personal opinion is:
Bad game design: You need to complete main quest, all side quests, exploration and companions quests to be level 6 at the end of Act 1, level 10 at the end of Act 2 and level 12 at 70% of Act 3. And if you miss something you are under level for next zone. So basically game is pushing you.
Good game design: For completing main quest you are level 6 at the end of Act 1, level 10 at the end of Act 2 and level 12 at the 70% of Act 3. And side quests, exploration and companions quests are here to makes your journey through main quest easier. Like that gives you at most 1 level higher, some items, potions, enchants etc. So they here as optional thing not mandatory. So game is rewarding you.
Also I sincerely would like to play with full custom party. Only think that I’m afraid of is missing experience from companions quests.
