I own the base game of Diablo 3. I didn't really like the "post-Inferno" content of the game, so I stopped playing shortly after I curb-stomped the final boss for the n-th time.
I understand now that a lot of things have changed in the base game. There's also an expansion. I don't plan to purchase the expansion, but some of these changes sound like they could make the game more fun for me.
I can guess that probably the new character and act are expansion-only content. I know I'm probably "doing it wrong", but if I go back and play now, what else will I be missing out on if I don't have the expansion?