One criticism people are raising, myself included now, is a lack of customization at character creation. Dark Heresy had Homeworlds, Rogue Trader had the even better Origin Path. These helped not only differentiate one character from another, they gave the character's depth and some mechanical advantages to go with their background.
In Only War, all the characters (Support Specialists perhaps excluded) are part of the same Regiment, drawn from the same Homeworld and put through the same Basic Training. This is good and right, because that's what being in the Imperial Guard is. But despite the Departmento Munitorium's best efforts, the characters are still individual human beings. So the question becomes, who were they before?
I, at one point, argued that this what the starting exp is for. However, with a little experience in making characters behind me, I see that it doesn't work that way. Because of the Aptitude system and the very high cost of buying non-aptitude skills and talents, that starting experience (still in theory more than practice) refines how you fill out your speciality, but it doesn't really, freely let you define who the character was before the Guard. A Weapon Specialist who was a Hive Ganger ought to be a little different from one who was a Manfuctory Laborer or a Schola teacher.
I'd like to propose another aspect to character creation: Background. Perhaps just a single skill, chosen from a short list (Athletics, Common Lore, Decieve, Trade). This would, IMO, help breath some life into the characters.