You can't play the Nightfall nor Vanguard Heroic Strike, choose a specific Crucible playlist, select a difficulty for missions (a level 2 mission stays as a Level 2 mission, and can't be raised to level 30+) or even buy non-cosmetic Vendor items (they require Legendary Marks, which are only granted at level 35 - which you need 'The Taken King' to get there).