/nerdy diablo 2 fan kicks in
I've never really been into runes, but what they do is if you get a certain kind of rune, say example, a Tal, Kal, and Bal rune (note, probably not real runes).
If you happen to have a 3 socket sword, theres certain rune-words you can create to make items extremely uber
Like, if it was a Bastard Sword that had +10 strength on it, if you insert (example) runes in this order: TalRalBal, you would create the TalRalBal Bastard sword, which, if good enough runes, can give you about 50 other stats like +1 to all skills, +255 hit rating, +3 amazon skills, and so forth.
Probably play Lord of Destruction if you want more player. Theres ton's of new patches with LoD. I wish they'd revert back to 1.02
So yes, get it, it's not just the characters, it's almost a whole nother game.
Also, you get one more Act after Diablo, Act 5, Baal.