Underground
One of the most common problems when trying to download Minecraft mods is that some people do not have ModLoader or Audiomod installed first. Another common problem is that incompatible mods are being downloaded.
In all technicality, the most common block in Minecraft is Air. In terms of solid blocks, it's stone. There might be a few dirt blocks in the way, but digging around mostly yields stone.
There is a lot of things in Minecraft, new things and secrets come up in updates, Herobrine is a mythical creature in Minecraft that is so called "Notches brother".
Any age can play Minecraft. However, the most common age range tends to be between 8-14.
You can find it in caves and on the side of cliffs. It is the most common ore.
Shallow pools of water
Humans are nowhere near being the most common creature on the earth. Depending on what you consider a "creature", you could think of microbes or bacteria... otherwise, you might consider the fact that insect life takes up a massive portion of the total number of living things. There are many, many more ants than there are humans on this planet.
Redstone Ore veins are found below layer 16. veins are most common on layer 8. There are 25 ores per chunk on average.Sources: Redstone Ore - Minecraft WikiOre - Minecraft Wiki
Coal is very common. Caves are a good source of coal, but if you are mining, coal is most common at y=48.
Most famous Minecraft is probably 1.1 version.
In order of most common to rarest: Coal, Iron, Lapis Lazuli, Gold, Redstone, Diamond, Emerald.