I like this idea VERY much! I'm going to learn Java, so that I can start modding Minecraft and add things that I've always wanted in the game. Unless someone else is already working on it, I'd like to ask your permission to make this into a mod for [1.7.3]!
Food poisoning sounds interesting, but doesn't really have much to do with the hunger system. Food poisoning is something that could be implemented even without a hunger system, perhaps as a random chance of damage instead of healing when eating uncooked food, or as damage over time which can be cured with some kind of item, perhaps a plant like you said.
This gives me a great idea for an awesome mod! Ever since the Acid GLSL Shader came out, I've always wanted to implement Psychedelic Mushrooms into the game! It would be fairly simple, I hope! Basically, the mod I'm thinking of making now would be this hunger system, w/ the food poisoning that RockBreaker suggested, except certain mushrooms activate the Acid GLSL shader and also give you food poisoning! It would be an awesome addition to Minecraft for us more mature players!
I really dislike arbitrary meters describing aspects to your heath that don't have any real place in the game.
Hunger should be something the player learns by instinct. When they eat, they get passive stamina boosts, or run 10% faster. Why show a meter. Just let the player learn what does what. And by learning the associations, their behavior changes to "eat fairly regularly" Also by showing a meter the notion of "Make believing" goes away, and becomes a chore. Anyone ever just make some food and eat it ingame because they felt "hungry".
If a player never eats (god I always eat to replenish my heath.. I dont need a stinking bar to tell me) Notch could have their hand tremble a tiny bit, or make a stomach gurgle sound effect once in a while. Japanime does that alot.
I like this idea. I don't like the idea of it being turned off in Peaceful mode. Personally, I don't like that we autoheal in peaceful, I would like 'Peaceful' to mean no fighting bad guys, but not just a modified Classic game.
I know a lot of ppl like the idea of thirst, but I don't. I think things like thirst and food poisoning is getting kind of silly (no offense meant).
I think it is TOTALLY necessary to have ALL such things optional if they are implemented - an on/off button. The reason is that the beauty of Minecraft is its versatility - it is all things to all ppl. From a monster/fighting game for testosterone laden jocks, to an adventurous exploration sim game for the Robinson Crusoe crowd, to an 'Incredible Machine' game for the inventing brainiacs, etc., with options to make it more or less any of these categories.
I DO like the one fellow's suggestion that there not be meters, but that you slowly discover you're more efficient/faster when you eat regularly - when the hunger option is turned on that is. That way, it is more in line with the other things in MC that you are supposed to 'discover' as you go (Unless you cheat with a walk-thru or something). I can see that being harder to program, however, than just an indicator of some sort, but that part is Notch's department.