How it will go down.
As of late I've seen several threads debating myriad multiplayer and mechanic speculations. I'm going to sort them out into the most logical sorting I can think of. I don't claim to know everything available about Diablo 3, but I have read everything I found, including a few things other posters didn't know. Anyways, here I go.
Hardcore:
Hardcore vs. Softcore seems to be a rather intense controversy. SC players can't see the point of losing their characters in HC, while HC players like to think HC requires more skill and is more prestigious than SC. Personally, I'm with the SC players, but not the way you think.
The truth is, there isn't any difference between HC and SC. None. You could simulate the experience by playing Softcore and then quitting if you die. Not much point, hmm? If I were working at Blizzard, I would actually make HC into a different mode. HC players would receive double XP and 1.5x items compared to Softcore. The rapid advancement combined with items steadily lagging behind levels would encourage Hardcore players to play fast and loose, to push push push until they die. Part of the fun of Softcore is getting attached to your character, something that is contrary to the idea of Hardcore. While Softcore is like an RPG, Hardcore is supposed to be like an Arcade game. You play for the high score, and when you lose you start over. In this way the two modes are appreciably different and offer actual differences in play style.
PK
I really cannot comprehend why people defend PK. Blizzard has designed their game so contrary to PK, yet some still insist on its inclusion. I define PK as non-consensual Player versus Player gameplay, ie Ambushes, Ganking, or surprise encounters.
The entire idea of a PK appalls me. I can understand the motivation, certainly. When I played WoW, I too engaged in a bit of ganking. It is definitely enjoyable to crush a low-level player beneath your might. But I always had a rule that I would never camp someone more than three times. There comes a point where the thrill stops coming from exulting in your superiority and starts coming from simply ruining someone's day. I once had my 34 priest get chased by a level 60 rogue for a good Ten Minutes before being cut down, and the enemy continued to wait for me to return. The whole time I was asking myself if this person had nothing better to do. Is it really worth chasing one low-level player for ten minutes? If the answer is yes, then you are either incredibly bored or a griefer that shouldn't be a part of a community.
The second argument I hear is that surprise PvP encourages realism. That's fine! In a game like Age of Conan or WoW, it reminds you that you are in a persistent world, and it can really get the blood flowing. However, there is a limit. If all you want to do is fight monsters, then constantly getting interrupted by an axe in your spine is just annoying. Granted, one should expect to encounter other players in a known PvP zone, but ones of similar level that could be competed against. It is an unacceptable infringement on a player's right to play the game if they are attacked by a foe twice his or her level, or in a peaceful area like a starting zone.
Now, this all assumes you are playing in a persistent world where random encounters are part of the fun. But Diablo is different. It is an instanced game, and one designed for co-operation. How would you feel if you were playing, say, Super Mario, and another player randomly popped in and starting killing you? (I know it's a weird example but there aren't many games without explicit PvP included).
Blizzard has limited the number of players in a game because they want people playing WITH each other instead of just NEAR each other like in Diablo 2, where a level twelve group of three would be in the same game as a level 39 sorceress doing a baal run, a level 30 going through the durance of hate, and three others just trading in town. Diablo is not a persistent world game, so they want people to join a game to play that game instead of just using it as a space to do other things.