Now, I love Minecraft. I love its look, and I've had a ton of fun playing in Hardcore Survival mode since its official release. I also love simply exploring the beautiful, procedurally-generated worlds.
Most importantly, though, I love how well Minecraft illustrates some beliefs I hold about games:
1. All games are educational
2. All games express values
I think that if you look really closely, this is absolutely true. The trouble with most games is simply that they just don't teach anything valuable and they don't express much about the world that we inhabit after turning the game off. Minecraft does, however, have some lessons in it, but they are ones that are unrealistic and, in my opinion, unethical.
When I use the word "realistic" I don't mean as it's used in the typical context of games. Using that word to talk about games will usually be in the context of bleeding-edge, photorealistic graphics and physics engines. The realism I'm concerned with is verisimilitude, how much a game resembles truth and/or reality. Very few games have much to do with the world we live in, even by means of metaphor.
On the other hand, Minecraft, as a simulation, carries a lot of real-world meaning. In the real world, humans mine, craft, build, farm, etc. It is in the specifics that Minecraft becomes unethically unrealistic.
If you consult the Minecraft wiki, you'll see that steak is the most efficient food in the game, now that you can breed animals. You can use three units of wheat to make a loaf of bread which will restore 3 hunger points, or two units to put two cows in (apparently lesbian) love mode, giving you another cow. This cow will later, when you kill it, give you between 1-3 pieces of raw beef or steak, if you kill the cow with fire. Even if you don't set the cow on fire, you can get steak by cooking the raw beef. A steak restores 4 hunger points. The cost between raw beef and steak is negligible.
So, to simplify:
3 wheat -> bread -> 3 hunger points.
2 wheat -> cow -> 1-3 steaks -> 4-12 hunger points.
So based on this, meat in Minecraft is 2-6 times more efficient to produce than (what I believe is) the cheapest vegan food in the game.
In reality, one pound of meat has been shown to be 16 times less efficient than one pound of non-meat food [Edit: Feel free to criticize this figure/source in particular, but the general concept is not that controversial]. Because Minecraft animals don't need food or water to survive, the simulation is flawed and, as someone interested in the real-life abuses of animals, I find the representation disappointing, to say the very least.
I'm a vegan in real life and try to play Minecraft without killing passive mobs like cows, pigs, sheep, and chickens. Though I do think the animals in Minecraft are adorable, I'm not so crazy as to think they have some substantive subjectivity like real sentient beings do.
If nothing else, it's a self-imposed challenge. It's not unheard of for people to try to be vegetarians in NetHack, so there's some precedent for this in a pure gameplay sense.
There is also a role-playing component to it, though. Though I won't argue about the ethical weight of digital meat, I will say that it isn't meaningless to make the decision to not kill animals (passive mobs, specifically) in Minecraft.
I'm not saying Minecraft needs to change, though I'd clearly like it to. I'm just using it as an example of how games necessarily express values by their rules. Minecraft expresses that meat is a better food source than plants, and this isn't true in real life. I'm not going to scream that Notch is a bad person for his unrealistically oversimplified model of livestock, but who knows: maybe someday I'll make a Minecraft mod that discourages meat-eating in realistic ways.
I do, however, applaud Notch and Mojang for making it so sheep drop more wool when they're sheared than when they're killed.
Edit: Here's a non-PETA article about the inefficiency of meat for those of you who simply think everything PETA says is a lie.