There will be no content patches (other then the one that is going to add the arena, which had to be delayed as to not push back the release date) content will get added in the next expansion only.
Diablo is... different then any other game really. The story is great, but you see the entire story in normal mode. I can't really describe what compels you to keep playing into nightmare / hell / inferno... its just so god damn fun. Its really just grinding, that doesn't feel like grinding. It's all about building your character and hunting down that loot. Finding rare stuff is such a rush.
Co-op is fun, but not required, you can have just as much of a good time by yourself. Personally i like to sit in my own game by myself but then chat to friends / guildies from other games in Vent.
PvP is purely for fun, and mainly just to mess around with your friends.