Also, as far as making diamond armor more common unbalancing drops, even with the much higher frequency in my mod I have not gotten many pieces of diamond equipment, and certainly nowhere near enough to repair all of my gear. I did nerf the drop rate of equipment, from 8.5% to 5% for most items (10% for amethyst), but Looting III increases that to 11% or 22% (unlike vanilla Looting acts like a multiplier, instead of adding 1% per level regardless of the base chance). And besides, with Mending and all I don't think diamonds are even that valuable; i even added them as a mob drop so they are renewable (actual diamonds, not pre-made gear, which is 100% renewable in versions before 1.8 thanks to villager trading, who sell everything. The only things that are not renewable are enchantment tables, jukeboxes, and fireworks, none of which you need many diamonds for. The mob in question is Giants, which rarely spawn (much rarer than any other mobs) and only drop them when killed with Looting; you might get 1-2 per night if lucky).
Here are the drops that I've gotten from over 3 months of intensive gameplay, with around 35,000 mobs killed, around 2/3 of those skeletons and zombies (ignore the diamond pickaxes with 100% durability, I found those in minecarts, from 53 mineshafts, the three iron pickaxes at the bottom were likewise taken from them and enchanted so I can use them to dig railways). Note that zombies have a 6.6% chance of weapons (6.6 times the chance in vanilla on Normal) and 20% of those are diamond or better; armor chance is 20% (on Normal, 30% on Hard, which is double the vanilla chance; actually, 1.8 nerfed Hard a bit by making the maximum (clamped) regional difficulty only able to reach 1 instead of 1.25, which meant the actual maximum chance in vanilla 1.6.4 is 18.75%):
Also, drops are not so useful when you can't even use them to repair your gear, thanks to the good old pre-1.8 anvil repair system (expensive? But you are not supposed to walk around with gear with 6-7 enchantments, never mind that Mending basically makes them unbreakable; my version simply replaces renaming to remove the prior work penalty so you must repair them on the anvil):
(I can repair my boots with a single unit - for 44 levels (anvil cap is 49 instead of 39 for these items); you do get 1124 durability back (all pieces of armor have the same durability) but that's still more expensive than diamond gear in vanilla, which can also get Unbreaking and be repairable 100% at a time using items you bought from a villager for a few emeralds/stacks of wheat, plus in vanilla I don't use Protection on my boots, which offsets the reduction in armor effectiveness for about the same amount of overall damage reduction, 76-77%)
Also, local difficulty (which again ought to be removed as I mentioned before) actually reaches a maximum after 16.6 hours provided you've spent at least 21 hours in the world (inhabited time is the biggest component but there is also a factor related to total playtime; what I'm saying is that they should make it only based on the latter, since otherwise I never spend enough time in a given area for it to have much effect, especially since 1.8; in my mod it takes 100 hours (75 on Hard) during a new moon. On average I explore about 100 chunks per play session, which lasts about 3 1/2 hours, so with a view distance of 10, which loads about 441 chunks, a given chunk is loaded for only 15-16 hours, half that time by the time I actually explore it).
So, that is what you get, mastercaver!