I thought that limited class options helped pick-up groups in The Lord of the Rings Online™ while making City of Heroes PUGs sometimes vulnerable to ineffective “concept” builds. Ravious commented that Guild Wars PUGs were improved by being able to see what skills other people were using, with the implication that you could educate the newbs or avoid people who look useless. (For the unfamiliar: you equip one bar of abilities in Guild Wars, and that is all you can use until you go back to town and change your bar. Many options, few available at once.) These experiences suggest to me that putting characters in small boxes lets you build things more easily.
Is this trade-off I am proposing between ease of grouping and narrowness of class roles a false dilemma? Can you have highly flexible character roles and still have the ability to assemble groups quickly? DC and Champions Online have both planned to bridge this divide by having roles that change on the fly instead of classes. That is, you build your character with “fire hammer,” “healing flames,” “fire shield,” and “blinding smoke,” and have a dial that makes you a DPS, tank, or support class. If you turn it to DPS, you deal more damage, but take a defensive penalty (alternately: set the dial to give only bonuses and not mention that the penalty is built in).
If the trade-off exists, which direction should your game take? I am a fan of individual expression, but as a game designer I would be more interested in seeing people play together frequently and smoothly. Bind people to each other, make cooperative play in your game a good thing, and let social factors help you retain players. This is why you add bottlenecks, downtime, and forced grouping: bring people together for interaction and bonding. Or maybe this is all crap, because WoW made its billions as an extremely solo-friendly game, but then the end-game is entirely raid-centric, forcing people into class roles.
Do you like being able to over-specialize? Planned supergroups in City of Heroes are awesome. Brutal Speed was a simple City of Villains idea: Corruptors with Kinetics, Brutes with AE attacks, everyone with Leadership, no one with knockback. A standard group was any 5 Brutes and any 3 Corruptors, with no further thought needed. No one needed to take Hasten or Stamina, because you always had 3 Speed Boosts available. And the little Leadership toggle bonuses stacked nicely when everyone had them. In The Lord of the Rings Online™, our Casualties of War group has low DPS because we all went with tanks and support classes. Why go for that soloing build if you will always have a group available?