I need games to have open-ended gameplay. Generally, when I look at reviews of games, I see how they were rated for gameplay and replayability. That's pretty much all that matters to me.
Some games do this well. I've played Pokemon Diamond for 350 hours, with only 15 or so of those being spent on the actual story.
For a game to work like this, you need
-The ability to replay challenging fights over and over again.
-A certain amount of unlockable content and side-quests.
-Online play, or NPCs that are fun to interact with time and time again.
A thing to be avoided: After defeating the end boss, do not give me the option to either start over with a certain number of my cool things or start from right before the final boss. Ideally, my victory over good/evil would be reflected in the world, and, for the most part, I could roam carelessly with perhaps a few high level dungeons or challenges that I can play over and over again for cool stuff. Rare item drops on extremely tough monsters encourage this sort of thing.