Character. Particularly the ability for a unit to have variation between armies.
I opened up the Adeptus Mechanicus Codex and got two very different experiences from the Electro-Priests and the Ruststalker sections. Let's go with the bad first.
The Bad
The Electro-Priests come in two flavors Fulgurite (Motive Force Takers) and Corpuscarii (Motive Force Givers). These are two differing philosophies, but the section is very definite on how Electropriests behave.
All Electro Priests have the following
Electoos
Omnissiah's Tears
Are either Fulgurite or Copuscarii
There is mention that these two factions were once at civil war with one another, but now it's mostly a rivalry with small groups of agitators actually fighting one another. But there is no depth. No mention of other sects that have different trappings to help spur creativity. There isn't even mention of a named Electro-Priest who I could use as a jumping off point for a creative character. Some people may say not having anything definitive allows for more room to explore, but I'd at least a couple of examples to get the creativity flowing.
The Good
Now for the Good. Ruststalkers used to just be Skitarii Assassin's back in 7e. But then they added one extra section that makes me appreciate them a lot more lorewise.
Legends Told In Binaric Code:
However, this has not stopped a few clades from distinguishing themselves, and if their deeds and heroic efforts are not recognised by their masters, they are at least made note of by their foes... Clade R2-111 attacked during the acid rain season, using the deluges to cover their infiltration.They developed distinctive dripping rust patterns that the greenskins grew to fear, naming them the Rust-rippers... Several clades from Stygies VIII have earned similar infamy amongst their foes, including those clades known as the Crimson Blades and the Skullmarks. These unitsseem to have gone rogue, often bearing unusual markings and appearing unbidden amongst other legions that have been summoned to battle.
This adds so much more variation on the Ruststalkers. I've come up with one of my own Ruststalkers who has been doing their own field repairs, but has since recognized that most of their body is no longer their original. Another group are made of Skitarii combined with animal DNA.