Topping the chart, at least for me, is the situation where some enemy chases you down and keeps you from mounting so you cant mount and run away but then they start attacking you once they catch up to you but if you try to fight them back they turn and run away.
Clearly they shouldn't get an option to run away when you dont get the same option. If they hit you and you hit them back, they should be obligated to stay and fight OR if they still try to flee, then they should not be immune to your damage.
THe second on the list is the cant mount in combat nonsense. As per the above mentioned passive agressive nonsense, if they can flee from combat regardless of if you try to fight, so should you be able to and mount if desired because thats the best way to flee combat really - burn up the horses stamina and not your own.
After 5 years of playing this game on and off i'm really fed up with both of those circumstances