The thing everyone seems to miss with non-human chaaracters is the roles they were intended to fill in the stories and games they feature in. Ogryn are the simpletons of whatever group they're in. They are written as flesh-and-blood tanks that are given comrades soley so they don't kill their own squads. They almost exemplify the guard itself, huge, powerfull, and by being stupid stay extremely loyal. Players could easily play one as uniquely insightfull Forrest Gump-style, or the literally-minded Slingblade-style retard. Also, thanks to how skills are set up in OW, dumb doesn't mean unskilled.
Halflings and Kender were both used in place of children and that level of innocence. They are meant to give contrast to the worlds around them and maybe remind readers/players a little of what they're fighting for. Tasslehoff and the Hobbits all grew up by the end of their journeys and could never really go home again. Warhammer makes this role problematic. The value set, in-universe, is different. Ignorance is a virtue, hate is a good thing, fear is encouraged. These are not the things people like to see associated with children and with good reason. GW keeps away from child abuse by treating Ratlings as a "joke" race, but a more fitting move might have been to describe them like Afflicted Kender, which are really just war-zone children (innocent, but morbid and violent because they don't know any other way), giving them a more nihilistic vibe than slacker-style. I don't blame FFG for sticking with the source material, but is does seem a little lazy to just copy them straight over.
Dwarves are the sad derivatives of Tolkien's creations in D&D. In Tolkien they're hairy, hearty and short because they live in caves in mountains that are all rock and ice. They forge because metal and rock are their most plentiful resource and they're greedy and insular because their race is in decline. The Hobbit is really just a last grasp at faded glory for them. D&D just took the outline and ran it into the ground. WHF breathed new life into them IMHO, gave them an edge with their grudges and hatred. Which is why it honestly surprises and dissappoints me to see them turned into goofy biker-caricatures in the tabletop and the pathetic Rogue Trader from the Ravenor books. Bring back the hate, hardness and long memories and I think they could fit in very well in any FFG 40K games.
The main problem with Abhumans, in OW anyhow, is that they've been mis-placed. The ab-humans should have been a Pre-made Regiment or Regimental World option, not classes. Using race-as-class is something from Ollld school rpg design that is generally considered a mistake nowdays, and doing that here puts them too much in the spotlight. Theres nothing that says a regiment must be all from one world and as a game progresses it'd make sense that the squad might get a little mixed, a Storm Trooper from a Schola world, a Priest from a Penitent one. Just refresh comrades from the core regiment's world and mention that elite troopers from other worlds sometimes get added to select squads, like the PC's.
Abhuman worlds could have had similar traits as the Penal Colony world to reflect the empire's negative views regarding them and the talents that were a part of the classes turned into race/world specific talents (regimental world based talents btw, hint, hint). Then you could have had snipers and heavy gunners that would more realisticly appear in a regimental tithe, coming from the same world. You wouldn't even have to restrict classes from them, just penalize starting characteristic modifires, starting aptitudes and the xp cost for playing a Commissar Ogryn would discourage all but the most determined player from even trying. The book is already out though, so maybe in an alternate rules book, web supplement or a future edition if they like the idea.
I'm totally writing up a Dwarf Regimental World now, with special bonuses against anyone calling them "squats".