An open world concept really only works for online MMORPG's, because the developers behind such games continue to expand on the world the players are in, creating multiple quests, achievements, side stories, etc. One group of players could spend hours on one quest, while another group is focused on other quests. The world continues to expand, new stories are added, and the players can more or less dictate how to proceed.
An open world concept does NOT just work in MMORPGs... Let's look at one of my favorite old time Space Sims, Freelancer. The ability to farm and explore unknown regions of space not used in the story was not only nurtured, but... wasenhanced by the engine. By allowing the basic explorations of the world, you may uncover secrets, power-ups and addons that while it was possible to play the game quest 1 to quest 100, you possibly could have won, you have a funner time blasting the boss with your more powerful artifact that wouldn't have been available had you not preformed those side-quests...
My point is... don't let the open roam too imbalanced... If its believed that at around 20th level you should be defeating this particular boss before going on, make a cap to the open roam, allowing the person only so much overpowering... Final Fantasy did something like that with the experience caps... a 1st level character would get a certain amount of experience while a 6th level character would get far less... Then by creating a choke point in the game, with more powerful adversaries only accessible on the other side of this quest, you ensure that it becomes harder and harder to farm until pre-requisites are accomplished. An example of that, is back in Freelancer, where you could not reach beyond Freedom Space until you received the security codes to enter Britonia space.