Allegedly, it's considerably easier than the TP map. There are still commanders with loads and loads of HP, but they don't deal nearly as much damage. The difficulty curve follows the order of the dungeons in Majora's Mask: south is the easiest, then north, then west, then east, which is about as hard as the TP map.
Removing a skill costs no less than 300000 Rupees. Okay, scratch what I said about good weapons being easier to make. Holy crap that's expensive. Now I really wished I hadn't plonked all that money in the Dojo. And I'm going to need to spend a fair bunch just to make Young Link and Tingle catch up, too.
New skill: Hasty Attack, which makes all your attacks faster. It's unlocked after 4000 KOs, and is probably going to be mandatory for any kind of ultimate weapon you want to make. Apparently "weak point strikes are stronger", "special attacks deal more damage" and "deal more damage at full health" are other 4000-KO skills that have been seen, but it seems to me like Hasty Attack would be better than any of these.
Young Link's weapon is actually the Fierce Deity Mask itself. All three levels use the enlarged Kokiri Sword, the aesthetic differences in the swords only kicks in as the Fierce Deity. Weird, I would've expected the Razor and Gilded Swords to make an appearance. Also, Tingle is Fire, not Water. Well damn, there goes my hope of ever seeing a good Water weapon. Especially since Fire is already pretty stacked itself.
For North Americans who want to get a head start in farming materials for Young Link and Tingle, Young Link requires Dinolfos, Gohma, Ganondorf and Ganon materials. Tingle needs Icy Big Poe, Link and Manhandla materials as well as Holy Hylian Shields.
Young Link's gimmick is that his C1 drains his special gauge to fill up his magic gauge. Since the Fierce Deity mode is activated through Focus Spirit, it makes sense that he'd have some way to fill the magic bar without pots.
The most worthless combo in the game has been buffed: you can now dodge cancel out of Zant's C5! In fact, there are early reports of Zant as a whole being buffed, but we'll have to wait for more concrete info at this point.
New potions: one that boosts experience points obtained for one battle, costs 50 bronze materials and 10000 rupees. Another one boosts Rupees dropped, costs 75 bronze materials and one Rupee (yay, you can actually do something with the dud Amiibo rewards!). The last one makes it more likely to drop rare materials, it costs 100 bronze materials and 50000 rupees.
The new Adventure map has item cards that affect the actual battle instead of the overworld map itself. The Mask of Truth doubles your defense for one battle (where was THAT in the TP map?), and Majora's Mask fills your special and magic gauges at the start of the battle.
If there's any more, I'll edit this post.
Edit: Apparently there's another new skill that boosts damage done in Focus Spirit mode. Screw "Darkness Warriors", the new title this game needs is "Fierce Deity Warriors". Also, I can't confirm this myself (obviously), but unlike 1000, 2000 and 3000-KO skills, you can stack multiple 4000-KO skills on the same weapon.