It's pretty damn simple when it comes to gauging a good game for me. Do I have fun playing it, and for how long?
With Fallout 3, I continue to have loads of fun, and see new things, every time I get back into the game. I'm tens of hours in now, working three save files, trying to play each end of the Karma spectrum, good, neutral, and evil. The simple fact is, I continue to have more fun than the last time I played it, every time I turn it on- there's something new to see on every inch of the map.
The details arn't important. As a fan of the original two Fallouts, I'm happy to report that the developer switch is not a dampener on the fun factor for this one. Sure, it could be a little more hardcore, but all the fun, and almost all the character, is still there.
If you buy this game you will have fun for a minimum of weeks, constantly. I swear it.