That's where I think most people are wrong about inferno though. That difficulty rewards proper communication and group play. Sure, soloing everything requires a crap-ton of godly loot, but damage and defenses can be compensated by a proper synergy of group skills. Lack armor and resists? that Barb shout may just solve it. Your tank getting face-melted a lot? You could have a monk stack a 50% damage reduction debuff on the enemy. Your glass cannon wizard getting chased down? Have a witch doctor load up on CC skills. What I'm seeing everybody try to do is to gear up and go berserker on the enemy and it just doesn't work.
On Inferno the monsters scale quite massively with each and every player that joins the game. I believe it's 110% of hit points and about 15% of damage per player.
So your party have to synergize alot in order to overcome this scaling. This is further exacerbated by the Nephalem Valor buff's mechanics - a bonus to magic find you accrue after killing hard mob packs - which prohibits changing your skills and runes mid-game. Hence you cannot really have your party members leave and be replaced by someone of different class without losing the synergy (or the buff).
In solo play, of course, you don't have these problems.
