mobs
Lots of stuff. Redstone powered lamps, zombies breaking down doors, AI for all mobs (makes mobs smarter), and many others
Passive mobs need light to spawn, and they only spawn on grass. If the aggressive mobs are spawning, then it must be night and the passive mobs will only spawn near torches, pumpkins, glowstone, etc. However, Notch MAY have fixed this in 1.6, but I'm not sure.
Mobs are enemies, so I suppose that there wouldn't have been.
No, mobs will not spawn in water or ontop of half slabs.
In Minecraft Classic, mobs can't spawn, so they can't exist naturally. If you play the Survival Test, and when mobs spawn there, you can save it and open it in creative mode so then the mobs can stay there. However, in Minecraft Creative Mode, mobs can spawn, but they can't hurt you because you have no healthbar.
no, some mobs can't, such as skeletons, creepers and zombies, but other mobs like spiders or endermen, climb over or just teleport in.
They are called troops.
lynching mobs
Yes, there will be other mobs! These include the Blaze, Magma Cube, and the Mooshroom. Also, other mobs will be added over time.
If you mean in all mods ever, then potentially thousands. Mods are add-ons to Minecraft created by users, and new ones are made every day. To count how many modded mobs there are in Minecraft would be near impossible
In Minecraft, a mob is an active object with A.I. (Artificial Intelligence). There are several different classifications of mobs in Minecraft. They are Utility Mobs, Passive Mobs, Neutral Mobs, Aggressive Mobs, and Boss Mobs. Utility Mobs are new to the game as of 1.9 Beta, when Snow Golems were added. Utility Mobs can be created by placing specific blocks in a certain arrangements. Two types of Utility Mobs are Snow Golems and Iron Golems. Passive Mobs are harmless. When attacked, they will not attack back, but may attempt to run away. A few types of Passive Mobs are Chicken, Cows, and Sheep. Aggressive Mobs are Mobs that WILL attempt to kill you, aggravated or not. Beware of Aggressive Mobs, many spawn at nighttime. Some Aggressive Mobs are the Zombie, the Skeleton, and the Creeper. Boss Mobs are new editions to the game, as of 1.9 Beta Prerelease 6. As of 1.2.5, the only official Boss Mob is the Ender Dragon. Note: Wolves can be tamed into pets with the use of bones, and Ocelots can be tamed into cats with the use of Raw Fish. Remember that Minecraft is always adding and changing mobs, so this information may not be completely accurate in the future. This information is as of 1.2.5 Official Release.