In theory, I am all for it. I can understand why they got rid of LAN in order for this to happen.
But now D3 has a few flaws that are preventing it from realizing it's full multiplayer experience potential.
1. Firstly, there is no UI design which allows easy chatting and forming of specific custom games. The party system is clunky, chat channels are hidden away in a tiny submenu.
2. Secondly, the main problem with multiplayer, is that the mobs get more HP.
the number of times that players aren't hitting the exactly the same mob together is close to about 90%. This means that essentially, you are still playing a single player game. But your monsters are tankier, and there are less of them to kill. That's not very fun!
In order for the D3 multiplayer experience to be truly what it was intended I believe they need to make these three changes:
1. Remove the HP bonus on mobs. It should be an incentive to party up to get to kill things faster. You can still make Monster Power make it still challenging if you want, but don't punish players for partying.
2. Add nicer chat functionality
3. Allow people to forego the match-maker and make a 'custom games' list. It doesn't have that much of a learning curve to figure out for the newbies, but the benefit you gain from it is insurmoutable. A custom games list lets people name their game, choose how many players they want. They can choose their monster power specifically in MP games, The benefits you get just dont stop.
D3 is supposed to be enjoyed online with other people. The current game lacks chat functionality, lacks a custom games screen, and buffs mobs HP. In order to really incentivise people to play MP they need to improve chat functionality, remove the bonus hp of mobs (monster power fills this role anyways), and add a custom games screen where a player can choose what kind of game he wants to join.