While I never played WOW, I have heard from some people who did that some people tried this once when the level cap rose, and gained a level or two immediately. Afterwards, apparently Blizzard stepped in saying "no no no" and rolled back those levels.
While I don't think having a couple quests to turn in immediately when 2.0 drops would be gamebreaking due to the limit on completed quests you can carry, it would be nice to know the answer to the question so no one wastes their time trying that.
I've only seen it worth two or three bars of a single level in WoW. I would expect it to work the same here, and intend to have a full list of completed quests to turn in when I log in to 2.0.