The party bonus is still there, just shifted over a bit. It used to be MF and experience; now it's gold and experience. It still stacks up 10% per member, so if you party with three other people, your gold and experience gains are multiplied by 1.30.
The other benefit to partying, which I find to be much more valuable, is that you can trade legendaries and sets to people within your party for 2 hours after the item drops. Other than that, all legendaries and sets are Account Bound, so being able to trade with the people in your game gives you a lot more potential finds, if someone wants to trade you for something that you found. In addition, it works especially well if you have a dedicated group of friends that you play with, because you guys can pass each other gear that would be better for each others' characters. For example, My friend Rory found a Blackthorne's pants that were no good for him, but were an upgrade for me, so he gave me the pants. A little while later, I found a Strength sword that was an upgrade for his Barbarian, so I gave it to him since I don't use mine, and he had already given me something. Because you can't trade anywhere else in the game now, trade between party members is especially important, so partying up is especially useful in loot 2.0/RoS. Just be aware that items tend to drop as Smart Drops, which tailor the stats to the character that finds them, so if a Barb finds a leg/set, it will tend to have Strength on it. The best answer to that is to have multiple characters in order to be able to take advantage of any kind of loot. This is made easier now by the fact that leveling from 1-60 is very fast: if you try, you can make it from 1-60 in an hour with no powerleveling at all, and if you get a friend to power you through, you can be at lvl 60 in less than half an hour. Thus, I would suggest getting yourself characters of all types, and a dedicated playgroup of friends, so that you can truly take advantage of party play. At least for establishing the group of friends, if you don't already have friends that play D3, the new clan and community support should do wonders for helping you to find other like-minded D3 players to party with.
