It's more a question of roles than classes. I've been in groups that didn't have a single Cleric, Wizard, Fighter, or Rogue, without a substantial change in power.
You need a healer. It doesn't have to be a Cleric, but under the core rules only a Druid can really substitute. The key is Heal/Mass Heal; as level increases the amount of damage per round increases until it's far beyond what Cure Critical Wounds can handle. A Paladin just isn't going to cut it, and neither is a stack of Cure Light Wounds wands.
Same goes for other needed roles: long-ranged damage, defensive "tanking" (get between bad guys and low-AC/HP types), utility spells, raw melee damage, and detection (Spot/Listen/Search) are the usual gaps to fill. In many cases, there's one class ideally suited to the role.
Frankly, any group that has each of these can do just fine. I was in an 8-man group of casual players; we'd have 5-6 people show up on a given night. It usually wasn't a problem, because of redundancy in key roles. We'd have to adjust our playstyles to match the attendance, of course.
(For the record, we were Wizard, Sorcerer/Rogue, Rogue/Ranger, Ranger/Bard, Fighter/Barbarian, Cleric, Paladin, and Druid... pretty nice distribution, huh?)
The problem, IMHO, is that certain classes can fill too many roles. It's not that Cleric healing is so much better than Druid healing, it's that Clerics can heal without tying up all their spell slots, AND wear armor, AND can use better weapons, AND have plenty of utility spells to choose from.