Ye, halfling priest isn't something I get for PQI Priests it's just Gnomes and Elves afaik (I might've missnoted something). Orc Crusaders... not sure, it's Halflings, Dwarves, Elves, Gnomes and possibly Goblins (not sure about gobbos). I'm not sure that exeplorer classes have any limitations at all.
But again, I'm still not 100% convinced the pool is fully fixed and not dynamic and related to playstyle prefferences in some way). As far as I remember it really used to be either picking dynamically or creating the pool depending on least played stuff, and "unlikely heroes" and "bonus bravery" are kind of designed (afaik) so folks who play them don't reach the PQI in a way which would make it streaky. Maybe they changed it in the meantime, but a lot of my PQI choices actually make complete sense in terms of what I've underplayed and overplayed historically.
Noise or no noise most of the stuff in the PQI is either stuff I've underplayed because I'd pick the strictly more favourable but simmilar stuff (Like Orc Paladins, since Human who doesn't show up makes a superior Pally), stuff I've underplayed because it was boringly powerful (Orc Wizard, Orc Fighter etc), stuff that just doesn't make sense to ever pick yourself (halfing warlord, gnome priest) or stuff that's just well rounded but I'd always pick something more extremely focused (Dwarf Sorcerer, Elf whatever, various gobbos). I could interpret the greater variety when it comes to exploration classess by having underplayed them relatively to other classes in general (although the default choices I'd pick for them don't show up at all). No humans is easy to explain in various ways, too. Not saying it's not a pre-set pool, It's possible that my least picked stuff is very close to what a pre-set pool would focus anyway, and a bunch of stuff I'm trademark good with is actually stuff I got bored of the PQI streaking and just played them a lot and got familliar with it on one playthrough or another (Goblin Sorcerer, for example, Dwarf sorcerer on this one).
But it doesn't look like my old PQI, I used to get a lot more Codex Happy monks back then, for example, this time it's mostly gobbos as far as the eye can see. I could even dig up an OLD playthrough thread with no picks and rummage around that to see what it was like.
Also Wizard is Orc, Dwarf, Goblin, - EDIT: Halfling
As for "too easy", heh, Orc Wizard and Orc Fighter are in there, those guys roll anything in ways that make assassins look like pacifists. You can play stright PQI and folks'll think you're cheezing like a fool ^^
I got them all on photobucket, I can just grab whichever class we want and we can see what we can get from the data to condens for wiki-crash-course
Let's see, I'll just pick one class at random. (eh, can't not home, internet where I am sucks, will do so later).