It's the first game + improved story mode + more characters.
If you enjoyed the first one, you will enjoy this one, and if you have to choose between the two, this one is the best in every aspect.It even has all the characters from the first game so you won't lose anything by not buying the first one.
Since is basically the same game with more characters, and I already reviewed the first one, I'll just say a couple of things that i did not like, other than that the game is awesome.
First: Some characters are rushed, and by rushed I don't mean technically speaking, I mean they were made before they got proper attacks in the manga.
So most of the post time-skip strawhats don't have their best attacks, as well as the new villains.
Second: If you're not japanese the multiplayer can lag a lot. But maybe that's just me being european.
And now to wrap it up, since gamespot is probably not gonna review this, I'll list the new characters:
-All the strawhats after the timeskip.
-Smoker
-Cobi
-Rob Lucci
-Doflamingo
-Shanks
-Marco
-Eustass kid
-Trafalgar Law
-X-Drake
-Caribou
-Hody Jones
-Vander Decken
-Shiki
-Vivi
-Wapol
-Enel
Yeah...I don't know why they added Vivi and Wapol, but the rest are very cool.