Dear Notch. I love your game. Why and how it's fun is a mystery to me, but even though it defies all logic, I enjoy it immensely.
That being said, your enemies need a lot of improvement in their design.
One of the basic problems is that they feel like the bastard sons of satan. Nothing they do feels fair. If people don't feel like the game is treating them fairly, they get pissed off and stop enjoying your game, which is what's happening with me. Even if the rules that the enemies abide by are consistent, if the player feels that it's unfair then it doesn't matter. In Nethack there are a shitload of enemies that are ridiculous and will rape you from the other side of the room. But, Nethack provides mechanisms for dealing with those enemies. If you get lycanthropy and you don't have a wolfsbane you can still pray. If those wands of death are pissing you off you can get reflective armor. If you're about to get raped you can always engrave Elbereth. Even with Liches you can always see them coming and have enough time to execute the best maneuver to deal with Liches which is to run. With Minecraft it's more of, "Didn't see that creeper? Sorry, you're fucked."
Let's start with creepers. Creepers make absolutely no sounds or give any other signal until they're about to blow up. At the end if your teaser the guy says "WHAT THE SHIT" and he is right, that bastard popped up out of nowhere and he had absolutely no time to respond. Even
Scott Ransoomar gets ticked at it, as I'm sure you've read before. I was deep in a mine once and I turned a corner and ran right into one with much the same reaction. If there's something that could be done about creepers it wouldn't be so bad that you get no warning, but there's really not.
The skeletons. In my first game I was digging down and suddenly found myself in a mob spawning dungeon full of skeletons. I think I could count on one hand the number of seconds that I lasted before I died. It took me six tries to recover my equipment from that room. I ended up digging underneath it and filling the whole damn thing up with blocks. Even then, skeletons kept escaping. They must have hacks installed on their client because their arrows are always perfectly accurate. They can shoot you even if you can only see a tiny sliver of their arm. The arrows go real fast so they're difficult to dodge and in close quarters you really don't have any chance. Running up and smacking them is a great way to get an arrow in the face.
Zombies and spiders aren't so bad, except that even with my best swordsmanship I wasn't able to take them without taking a decent amount of damage. I'm honestly not a very good video game player, but even with practice I couldn't get the hang of what the trick is to killing them without getting hit. Health is at a premium in this game, and not being able to preserve it is frustrating. It also didn't help that when I did die, I'd have to find and trek back to my home, and hopefully not encounter
another enemy on the way there while I'm defenseless.
Generally for any mechanic that the enemy has, you want to provide the player a way to counter it. If the player has a way to combat the menace then he's less inclined to stop suspending disbelief and blame the developer for the crappy game design. For example, for the aimbot hacking skeletons, provide a shield, or make it so that if the skeleton gets hit they get stunned and can't fire for a moment. For the sssssssssboom's, perhaps give the player an item that warns when they're close. Now the game becomes fair, since players have a mechanism for dealing with the enemies.
I don't know about your personality as a game developer, so I don't know if that stuff is intentional. Some games make unfair enemies and mechanics intentionally. If the guy who made "I Wanna Be The Guy" read this he'd probably laugh at me. If you want to take the "tough luck" attitude then that's fair, but not many people play "I Wanna Be The Guy." A lot of people play Minecraft, but hey, I gotta say what's on my mind.
Anyway I'm sure you're immersed in suggestions, but here are mine:
* Make the skeleton arrows not always perfectly stupidly accurate.
* Give the skeletons some kind of warning when they're about to fire, like a sound or something.
* Give the player a mechanism for dealing with each type of enemy. Warning for creepers, defense against arrows and so on.
* Except for maybe the creeper, use audio better to announce enemy presence.
* If you hit an enemy, stun him -- he can't fire for a moment. So, if the player gets the jump, he doesn't have to worry so much.
* Let the player set his spawn location so he can spawn in his home.
And lastly, try to think some about the strategy for spawning mobs. Is this a "tower defense" style game where the mobs come in waves? Or do they trickle in one at a time? Do they spawn anywhere or only at certain locations? When can the player consider himself safe? I think if you sit down, think about the design goals of the game, and work out a consistent strategy you could improve it a lot.
Sorry for the novel. I hope it helps.