What makes a great RPG board game?

1st: The Story Line of the Game.

2nd: Simply how you create this RPG Board Game.

Often people use a complete predetermined story line, whereas others create only the base for a story line.

Creating a story line that is set in stone can create many problems. You want your players to have fun and feel like they have full control over their character.

That is why I recommend creating a strong yet flexible base story line, so that your players can have a small or large effect on the game's outcome. That way, players will be more involved and inclined to play. But, like I said, your base story line should be strong, extremely well rooted and yet still flexible.

Key Items: A Vast and Interesting World, a Complex Story, Interesting Characters (each having their own story), and a well thought out battle system.

Well, RPGs are about two things:

1) progression (you gain levels, stats, powers, abilities, etc); and

2) customization (you personalize your hero by making choices).

So a RPG boardgame should incorporate these two aspects. Also, since it's a board game, you need to keep track of rule complexity. It should be simple so it doesn't take more than 2-4h per play session.

Last, random elements like a random adventure path through a landscape could create replayability, where people return so they could explore additional aspects of the game. Random enemies that require different types of heroes work great too!