There are a couple factors to consider when upping the number of players in a group.
First, you need to consider the graphical demands in play. Will the majority of computers have the system specs needed to process a game with more than four players acting in the same instance? After a few years, this restriction may decrease as the technology of the average home computer improves.
Secondly, unlike in WoW, when a new player joins a game all the monsters become tougher despite still being the same level. For each character beyond the first Blizzard has to properly balance monsters to remain challenging in relation to the number of players wailing on it. Sadly, there is no simple way to mathematically scale up the difficulty. Damage Reduction, Damage Output, Resistances, and lots of other stats all interact in a non-linear way with each other. Just because we add four more players to the game doesn't mean the monster's damage output should increase by the same increment for each extra player. And when we consider how resistances and armor and other stats synergize, the challenge of balancing monsters becomes progressively more difficult with each new player added.
Balancing the difficulties of monsters for groups of 1 to 4 players is an excellent start for Blizzard. If they can finagle the development time to balance each monster for a fifth player that would be awesome, but players should not make the mistake of assuming that doing so would be easy or quick to accomplish.
Maybe they shouldnt have added lame final fantasy graphics to every attack and left it like the Diablo series has been.
Yeah... as 2D sprites...
oh wait. That'd be worse.