You kill Baal, Tyreal destroys world stone, the end... not much of a story is it?
However, what you really need to know is that no matter how many expansions are released and such, Stone of Jordans (SoJ's) will be THE currency.
How did it come about, well way back in D2 V1.03, a bunch of ppl found out how to dupe and such, and well, they mass produced SoJ's and Dual Leach rings. The duping may have come to an end, but there are still ppl out there with 30+ SoJ's and they influence the econmy with them as SoJ's are now even harder to find than before. Dual Leach rings could have became an alternative to the SoJ currency, but I don't remember why it never really took off.
So the story of D2 is, SoJ's SoJ's SoJ's
The End