So just what was Belial's plan there?

[Roland]

At that point, Tyrael's not the Aspect of Justice anymore. He's not even an angel anymore. He's a human with a powerful sword. But he probably doesn't have the power to destroy such an artifact with a swing.

Nope, he's still the Aspect of Justice. That's why the dead rose around New Tristram - their anguish at dying without justice raised them when Tyrael hit the ground. Also, he clearly still has his power with his sword as he uses it to clear a path in Act III and Act IV.

The story is absolute shit. There's really no denying it. It's terrible, terrible writing. All the same, I enjoyed the hell out of it my first time through. The only twist I missed was the Fallen Star was an angel (once I knew that I knew it was Tyrael, and every other "twist" I knew right away). Hell, half of us knew Leah was going to be Diablo months before the game even came out! If you choose not to get so hung up on the story it's a fun little ride. If you nitpick it to death you're only hurting yourself.

I already have a 50/50 shot on what the story's going to be for the expansion (one of two ideas, or perhaps both ). Doesn't mean I won't buy it for the story. Not all stories have to be great to be enjoyable.