The loot system is maybe one of the most difficult to balance. For sure, there should be personal loot for everyone who made damage to the creep but there should be also something for the healers who don't attack for example. The personal loot can be coins/gold or whatever is the main currency in the game + some common type of items. Lets say if the items in the game are rated as common, rare, epic, legendary, the personal loot can have up to epic items and the legendary items are only for roll. Here we can have another issue - how will be able to roll for an item? Only the ones which made some damage? What about the healers? Also, what will happen if people not in party or from different parties killed together a creep? This can be avoided if legendary items can be obtained only from dungeons and every party has it own separate raid.