While there are 2^64 or 18,446,744,073,709,551,616 possible seeds most world generation can only take advantage of 2^48 or 281,474,976,710,656 possible values, meaning that every seed is one of 65,536 that will have the same world generation wherever biomes happen to be the same (the biome generator is the only part that makes full use of the 64 bit seed. Non-biome dependent features, such as most ores and caves, and the underlying terrain height variations (modulated by biome height) will be the same everywhere; the locations where structures generate will also be the same, so a plains village in one seed may be a desert village in another; prior to 1.13 (which split "temple" structures into separate structures) a desert temple may instead be a jungle temple, witch hut, or igloo):
Another interesting fact that sources like the Wiki and even Mojang themselves (based on replies on bug reports like this one) get wrong - caves do not directly cut through anything other than the terrain itself, as they are generated in the same step, as opposed to "decorations", including structures, which only fake being cut apart in the case of e.g. strongholds by not overwriting air (which is also only for the walls, and not at all for the portal room, so claims that an end portal was destroyed by a cave or ravine are completely false. Changing a single line of code is enough to make them generate fully intact).
On the mobs skins, they have (on a couple of them, e.g the pigman) a signature on them, which is supposedly the creator of the skin.
On the "10 Years of Minecraft" map, they're loads of easter eggs! At the supposed end, there's a banner of herobrine. Also when you ride through the hall of updates, at the igloo it say "Mukkis" What the heck?
The old leather armour texture was re-used from one of Notch's (Markus Persson's) other games.