It is all about freedom. Your character is simply not restricted to any zones based on their level. There are no 'low level zones' anymore in the game, every zone is scaled basically to CP 160. You now have the freedom to enjoy the game how you want, in particular enjoying the story lines of the quest zones at a pace you prefer without fear of over-leveling of under-leveling the content. Finally, One Tamriel shines in your ability to play with a friend of any level (higher-lower) with you, or they, completely trivializing any of the content.

Try not to fall into the trap that many do though regarding 'competitiveness'. At a low level your character is still significantly un-powered compared to one that is high CP. This is due to learning skills/morph, having skill trees progressed (e.g. fighters, mages, undaunted, racial) and most importantly, having the skill points available to invest. Not to mention the increased skill that comes from actually playing your character and getting use to the skills/how they fight, rotations etc etc. Leveling in ESO is so much more than just that number next to your character's name.

At the heart of it, ESO is a quest-centric MMO, and One Tamriel provides the enhanced freedom to enjoy questing in a capacity you choose.