Related to this: http://paizo.com/threads/rzs2rtpf?Thought-Experiment-Single-Class-Campaign
But did not want to go off the OP topic or derail on a tangent.
I had always wanted to run a themed series of adventures (or even full campaign) where all the PC's were of one class or one race where that part of their capacity was instrumental to the campaign itself.
I can imagine an all-PC paladin warparty, or dwarves, or a bunch of rogues finding a town has no real guild and forming their own to run the place, wizards out to destroy magic item XXX from the world or seeking it, a band of gnomes trying not to bore themselves everywhere they go, etc.
Any Players or GM's try out just this type of thing and how did it work out?
I once made a campaign where all the players were human children from a small Midwestern American town who get magic powers (ie class abilities). Ironically it was being forced to play a kid that caused the most grumbling at first, though they all loved it once they started playing.
As for only allowing one class, I would only do that for a one shot, but even then I'm not really interested in that. Class is where players get the bulk of there game play tools, forcing them all to have the same basic tools can get quite dull and also prevents some players from getting a niche to fill. While some classes have deverse enough archetypes to get around that problem a bit it still is quite restrictive way to play mechanically. Basically my advice to any GM looking to try this is to pick the class your using carefully. All rogues for example I can see ending very badly :p
I had an amazing all dwarf mini campaign/ adventure - mid level heroes were assembled for a very dwarfish story line of reclaiming an overrun mine. 4e though, kinda campy, all bad Scottish accents and one kinda Russian.
There was also an almost all paladin adventure into an undead realm, archetypes and feat selection helped a lot. Very epic. Non trivial deaths. Basically you needed paladins because anyone else would have died.
I'd recommend more of an idea theme then strictly class. Maybe you have a magic acadamy, or something, from which set out a wizard, magus, bard, etc. While the above knocks an all rogue party, just don't have them take on missions an all rogue party would consider suicide. Maybe they're an assassin team, and the loves security includes animagic fields. Maybe they're just all thieves. Etc.
Monks are another good one, easy to have pinching and archery for damage, sensei / flowing monks are good support/control, etc. Good for a mystical but perhaps lower magic setting.
Divine agents is perhaps the most mechanically powerful, able to do basically everything any party can. Paladin, inquisitor, cleric...a desnan can even scry and fry with the best. Etc. Just a. Few ideas that I've had fun with. :)
I'm currently in a gestalt game where one track has to be synthesist summoner; the idea is some people can integrate with outsiders a la Shaman King.
The synthesist thing meant that we could just dump our physical stats, but we were warned that some places have a stigma against spirit-users and that we should anticipate moments when we'd have to rely on our own stats.
It's working out well so far, the one main issue being that our AC was growing disproportionately to our HP (such that appropriate CR creatures couldn't hit us regularly, but creatures that could would deal too much damage in a single hit), but that's been resolved by docking eidolon AC bonuses.
Everyone the same race is a lot easier I think than everyone the same class. Most core races are good for at least five different classes.
I think in general the theme should be more of a setting concept rather than a mechanical concept. The theme might be "courtiers", and under the hood you might see more than one bard.
By the way, the Pathfinder novel "The Dagger of Trust" is about an all-bard party, and it's a lot of fun.
Themes can also take on some thing more subtle such as mood, or certain consistent signs used to forshadow events. (a black leaf that shows up at ominous events) They don't need to be as straitjacketing as forcing all players to be one class.
Themes are best subtly imployed, not jackhammer forced as a one class arbitrary straitjacket would be.