-Do more things like the "zombies can break down doors in hard". THIS actually makes for difficulty- mob intelligence. Preferably, make zombies able to break down doors in normal.
-Add more mechanics that are changed via difficulty.
-Make normal harder than the current game, and hard truly difficult. Easy should be about as easy as it currently is (which is actually very easy).
-Give mobs a lot more abilities. Zombies that can turn villagers into zombies, make zombies not de-spawn in hard, allow spiders to hang upside down, etc. Then adjust whether these abilities work or not depending on game-mode.
-Have more unique, biome-only rewards. Things like ingredients for new potions- USEFUL new potions. Hell, lilypads could be used for a "Potion of Water Walking". Things like this make biomes useful to travel to.
-As per above, more biome-specific mobs. Not "SssSand creeper (:", or mummies that are zombies with a different skin, or something equally dumb. I mean a mob that's really different from the other ones still, and give unique item drops. Snakes that are the size of silverfish, but can poison you, and only attack you if you get pretty close to them. Sharks that only spawn in ocean biomes, and drop shark fins used for... some water-based potion. Things like this. I could go on for hours, but I'd rather make a mod to show how it can be done.
-Make trees taller. Seriously, they look like crap. I've already doubled the height of oaks and birch, and they look great (like they always should have looked). Pine/spruce look alright, though.
-Make a proper pre-requisite system to biome generation- right now, it's pretty much entirely random where biomes spawn, so you have taigas next to deserts, etc. They should use a flowchart of sorts- getting which biome is there, then deciding which biome(s) to generate around it depending on whether or not it would be appropriate to place it there. This wouldn't be too hard to implement, and would make things look a lot better. This also relies upon more biomes being put in (for more "transitional" biomes, like savanna between plains and desert), but that's kind've a given.
-As mentioned above, more biomes. Not just for the sake of having them, of course- but by now, that should also be a given. Things like Savanna's, Bogs, Marshlands, Hills (Just hills, not extreme hills... which should be renamed to mountains, anyway), Badlands, and plenty more. All of which have some use, and all of which should look different.
-Variations of existing biomes. Things like spruce forests (not just in taiga's anymore), rocky mountains (all stone instead of grass/dirt), forest mountain (think eastern United States) and so on would be nice. Variations of stone types would help, as well- brown, dark grey, etc. Similar to the variations in wood.
-Make plains more flat. Right now, they're basically hills, and that's a little silly. Make a hills biome, splitting it from the plains biome.
-Deserts shouldn't have pools of water in them. This is just stupid. Players should have to use desert wells for water here.
-Add in "Knockback" as a weapon stat. At the same time, add "knockback resistance" to armor, with heavier armor having more knockback resistance. Mobs should also have varying levels of knockback resistance- skeletons could be easy to knock back, but zombies would be harder. Generally speaking, it should be harder to knock things back than it is right now.
-Add an attack speed stat for weapons. The fastest should be the default attack speed (which is 3 per second IIRC). Pretty straightforward-
-Make new weapons. Battle axes, maces, spears, crossbows- all of that, and preferably more. Give them unique levels of of those 3 stats (Damage, knockback, and attack speed). Then, give them unique secondary abilities (right click)- spears can be thrown, maces can stun, and so on. Maybe even make different weapons have another ability- spears could hit 1 block farther than regular weapons, etc. Tons of ideas, and as long as they're balanced, they'd be fantastic.
-Increase default sprint speed. Then, divide armor into different "armor classes", or assign them different "weight" levels. The better the armor, the heavier it is- heavy armor would make you sprint slower, and after a certain point, you'd be unable to sprint with really heavy armor. It should also disable certain other abilities (again, more on this later) after certain points. This would make going without armor fairly viable- just the same, going in full armor would mean you'd not be knocked back nearly as easily (and plus still be a juggernaut).
Honestly, though some of his ideas are bad, a good lot of them are actually very good. It just seems like a large amount of players here are afraid of change in Minecraft. Honestly, the game is piss easy as it is, and more aspects and complex gameplay would really make it more fun. I mean, everyone begged for Jeb to reintroduce the complex brewing, but nobody likes the idea of more complex and rewarding combat and mobs that are actually scary? I feel like you're just bandwagoning on this guy now.