The first play through of Diablo is about the story, the second, third, and now the fourth play through is about the challenge and getting better gear. The end goal for most is either to do this over and over again, hunt for loot, or to PVP. However PVP is not going to happen at release, but this does not mean we wont need to prepare for it.
The game will not be for everybody, but it will be an amazing game. More than likely you will have fun with it. Also to answer your question, the expansion will probably cost $40 - 50 apiece, which will probably add more classes, acts, and new features. I would suggest trying Diablo 3, and before hand read up on what has happened before this game. The actual books are full of good lore, and reading up on what happened on the game is intresting.
Without experiencing a Diablo end-game, most people won't appreciate the theory behind how it all works and why its so fun. But it is awesome. Items in Diablo 3 are far more interesting than in MMO's, and offer infinite upgrades. The whole system is built around providing an enjoyable experience for years on end. Many would say (including myself) that the Diablo end-game is far better than any MMO so far.
The goal of every Diablo game can be boiled down to one word. Discovery. You reveal lore, you find adventures, you uncover strategies to get better, and you find loot. People will say alot about killing monsters, but the main reason is that loot pinata to see if you have found something awesome. Everything in Diablo leads to some sort of discovery.