According to a post on Reddit by a seasoned beta player, "You will be able to continually progress by gaining skill points to eventually learn everything, try new builds, take on new roles and play styles. Your character will not be simply a Rogue or a Paladin static and unchanging. There is a LOT of build diversity."
Along with that, the leveling system is much like that in Skyrim, where both passive abilities and specific skill can be leveled.
In terms of quests, while they may be bland, beta players have repeatedly said that The Elder Scrolls Online is less about quests and more about exploring the world in front of you. That combat, while simple to start, gets much more in-depth, requiring a party to tackle most dungeons.
Dungeons that you'll want to explore mind you.
While I've had two invites to the beta I've declined them both. Unfortunately I am not able to give you any feedback. I want my first impressions to be release day!