Tougher monsters, more loot and also this il qoute "Being in an area with a party member grants you a 30% experience bonus". But yea, sofar ive avoided grouping up entirely, since I dont want anyone getting dibs on my magical pants.
PS I also found this in the homepage:
Ironman
In Ironman, players are unable to trade with vendors or refill their flasks in town. Mana regeneration is disabled by default. Players are encouraged to band together and manage their resources in order to survive.
Cut-throat
The cut-throat leagues appeal to the niche of players who demand the most hardcore gaming experience possible. In this mode, all world areas are public, with full PvP enabled by default. Slain characters drop all their items upon death. This game mode can also be combined with Hardcore to create an even more unforgiving world.
Attrition
An attrition league is a short duration league that starts with a fixed number of players and eliminates the player with the lowest amount of experience at periodic intervals. For example, a 24 hour attrition league might start with 100 players and eliminate one player per 12 minutes after a four hour initial leveling period. Players would be able to see how close to elimination they are on an on-screen ladder display.
I think this is all planned. Pretty sweet stuff imo. Also Act 3 is still in the making.