What Makes an Army Good?

After posting about my most recent (and short) battle with Orks, here, I got some strong remarks returned about the fact that Orks really don't suck. The argument was that their ability to bring loads of models and consequently throw loads of dice means that average probabilities eventually win out yielding results.  On paper, I totally agree with this.  In practice, it's a bit tougher though. 

Anyway, before I go off on a tangent and rehash what has already been said, this got me thinking.  What makes an army good or bad?

Is it its competitive ability?  Can it stand up in every phase of the game and give a good showing?

Does it have several strong builds? Can it put together mech, foot, and hybrid lists with equal success among each?

Is it fluff/character?  Does the army have a distinct flavor that makes it fun to play?

Is it simply the models?  Beautiful models make me want to start many an army (though a shallow wallet keep me in check).

What is it, community?  Leave a comment and let me know what you think makes an army good or bad.  Also, what armies do you think are bad?  I think we would all agree on some, while others may be up to personal taste.  Finally, I am adding a poll on the right.  Mark your main reason for liking/picking an army.