None of the ideas are really new, but I'll give some reasoning as to why it's not implemented.
Emerald Tools - Emeralds are supposed to be used as NPC currency, if you turn them into craftable items you give them a different value. Right now, diamonds are the most valued item if you log onto a server, but if emeralds are better than diamonds, AND are able to be traded with villagers, you've messed up the order of things. Emeralds are a lot easier to get than you think. All you have to do is find a villager that gives a good deal or trades for something cheap like wheat. That's why they will only be a currency
Dual Wielding - It's just hard to implement. The obvious answer is to use left-click for left tool, right-click for right-tool. Well, right-clicking has a totally different purpose and it would be hard to just change the way things have worked when they are working fine.
Wooden / Stone Armor - I don't understand your reasoning behind this one. Iron armor is already pretty easy to get, especially when you find a cave system. Leather armor is more difficult to obtain in my opinion yet is weaker. Why would they implement wooden and stone armor that's almost too easy to obtain, yet make it more powerful than leather. Minecraft's health system is out of 20. The armors are balanced pretty equally. If you introduce a new system, you throw off a lot of the balances.
More Mobs - I don't really have a problem with that. More mobs are always pretty cool.
