As far as I know, difficulty (combat) only affects enemy troop strength.
So on higher difficulties you will fight more enemies. I would assume enemy composition also changes slightly.
So instead of fighting lets say 7 thugs on Normal you would be fighting 4 Raiders and 4 Thugs on Expert.
I am not sure if it affects “elite” spawns like Leaders, Hedge Knights, Overseers etc. but I would assume that you will find those enemies earlier on higher difficulties.
The Devs have stated that difficulty settings do NOT change enemy perks or stats.
Ironman also has no effect on difficulty/drops whatsoever.
I dont know about famed spawns at all, but I would also assume that difficulty does not affect those.
Generally, higher diffculty means more enemies for you to fight. This also means that you will get more and possibly “better” loot – if you fight more enemies, more of them will carry “better” weapons like Longaxes, Pikes, Warbrands etc.
If you would fight a Leader + Hedge Knight on Expert, you might only face a single Leader on Normal and potentially miss out on Coat of Scales. You will also not get rekt by the Hedgie though :).
Hope that answers your question.