Leaders still have traits, so that has not been changed. I don't know if stuff like cruel/sadistic impacts combat in CK3, or if it is now only the commander traits you have to watch out for. Levies were blobs of troops in CK2 as well, since you had no way to specialize what types of units your holdings were producing, much less those of your vassals. Perhaps you could shift it a little by building all cities or all temples in your realm, but overall levies would always be a mess of troops unable to pull of any coherent tactics.
Men-at-arms are much more interesting in their diversity than retinues ever were IMHO. In CK2 they would have a % bonus to attack or defence, which would give you either offensive or defensive heavy infantry. In CK3 men-at-arms have their bonus tied to terrain types or countering specific types of enemy men-at-arms. Gone are the days of camel cavalry spanking the Christians from Egypt to Cologne.
But, to be fair, combat itself is simpler now. You cannot manipulate flanks to place fodder in the middle and highly trained retinues/men-at-arms on the flanks. In CK3 martial impacts advantage directly, along with a dice roll, while in CK2 martial would nudge the notoriously fickle tactics system which was again largely a dice roll. Unless you were creating advanced retinue-only stacks that could reliably proc only favorable tactics.
Of course the AI in CK2 could not actually utilize this complexity, so it remained mostly a tool for the player to make combat laughably one-sided even against much larger opponents. I shant miss all the micromanagement involved, nor having to consult spreadsheets to figure out how to make my huscarls perform a shieldwall and not just stand around taking arrows to the face. Apparently you cannot shieldwall unless you have archers behind you plinking away. But not too many archers mind you, otherwise the huscarls will be too busy cheering on their own archers to raise their shields.
