For example: there are no Monks in the party, and I rather suspect that there will be very few in the world. I'm going for a more classic fantasy medieval Europe feeling, and Europe has very few traditions of monks who do massive damage with unarmed attacks.
I'm also planning to have few Barbarians, at least in the semi-civilized lands that most of the early campaign is focussing on.
Some other things, then, will be very rare--and this might even include PC qualities.
- Gnomes, I think, are going to be fairly rare--only the biggest cities will have any sort of gnome community, and there will be few gnome communities.
- I've got a half-elf in the party, so there should be some elves in the world, but I'm not sure how much I need dwarves.
- Sorcerers also are going to be very rare. Since the spells are more or less innate (compared to those of wizards, at least), there's no particular need for there to be a community of sorcerers to support each other. And from the journal entries Monica has posted, Larissa has grown up as a girl with very exotic powers--it makes a certain amount of sense for her to stay exotic.
- Rangers will probably also be exotic. Paladins too, but I think paladins will be more familiar in legend than rangers are. (At least, to be identified as paladins. Rangers may just be identified as "damn, that guy is *obsessed* with killing orcs", which is not particularly what 'ranger' connotes to me.)
Most prestige classes will be exceedingly rare, if not entirely absent--I'm not yet convinced that I want to use prestige classes at all.
The NPC classes listed in the DMG, though, will be exceedingly common. I'm sure that no one will be very surprised that the world includes plenty of commoners.