We know that the Warcraft 3 storyine takes a lot of its structure from Starcraft, but the order of the terrain takes a lot from Diablo 2. Compare:
Diablo 2: Act 1 takes place in a settled area, with some farms, buildings, and forested areas where it is not settled. Warcraft 3: The human and undead campaigns take place in Lordaeran, a partly settled, partly forested area.
Diablo 2: The end of act 1 is a monastery, a religious, possibly magical center. Warcraft 3: The last couple undead mission take place in the magical center of Dalaran.
Diablo 2: Act 2 takes place in a desert. Warcraft 3: The orc campaign mostly takes place in a dry area, partly desert, partly dry grassland.
Diablo 2: Act 3 takes place in a jungle. Warcraft 3: The Night elf campaign takes place in the dense forest of Ashenvale. (O.k., this one issomewhat weak, but it gets much stronger in Frozen Throne.)
Diablo 2: Act 3 takes place in a jungle. Warcraft 3: The night Elf frozen throne campaign's early and middle missions are in a tropical jungle as well.
Diablo 2: Later parts of Act 3 are in the ruined city of Kurast. Later parts of the Night Elf campaign, and early parts of the Blood elf campaign, take place in the ruined city of Dalaran.
Diablo 2: Act 4 takes place in hell, which is accessed through a portal in the previous city, is in another dimension, and the upper regions are composed of slabs of reddish rock floating in space. Warcraft 3: The later blood elf campaign takes place in Outland, which was accessed through a portal, is found in another dimension, is mostly composed (in warcraft 3) of reddish slabs of rock floating in space, and is also full of demons.
Diablo 2: The last part of act 4 takes place in the center of power of the main bad guy of the level, which is a blackish, with some reddish, building, with several skull piles sitting around. Warcraft 3: The last Blood Elf mission takes place in the Black citdael, and dark brown to reddish colored center of power of the main demonic ruler of outland.
Diablo 2: The first few levels of Harrogath take place in an somewhat sold area with lots of destroyed structures. Warcraft 3: the Sylvanas levels, and first couple of undead missions, take place in the destroyed lands of Lordaeran.
Diablo 2: Most of the Harrogath level takes place in Icy areas. Warcraft 3: The end missions in the undead campaign take place in an icy area.
Why do Scourge Meat Wagons deal siege damage, and in fact how does throwing a squishy corpse at a building deal any damage at all? It makes a little more sense when you consider that in ancient times, throwing plagued corpses over the walls at a city's defenders WAS a valid siege tactic, as an attempt to weaken or wipe out the defenders with plague. Still doesn't explain why it makes the buildings explode, though.
No reason they can't stuff the corpses with explosives.
Archimonde was killed by wisps, which can only deal damage to magic. Archimonde was trying to absorb all the world's magic, so it backfired.
Additionally, a Wisp's detonation damages summoned units in-game. Archimonde was summoned into Azeroth by Kel'Thuzad.
In the Stratholme level of W3, there is a brilliant race/boss battle(s). Typically, players will kill the enemy hero first because its the strongest enemy, but that tactic blows up in the player's face on the Stratholme level. Both sides are trying mercy kill/turn undead the villagers. Since every time you kill the boss, the resurrection timer starts and the boss respawns with a more powerful army. If you kill him first, that leaves you fighting his support troops and eats into his resurrection timer, leaving less time to mercy kill the villagers. Smart players will kill the boss last in the first few encounters, and only kill him first when its near the end of the race or its the only way to beat the enemy army.
In the Reign of Chaos campaign, in the first mission (assuming you did the sidequest), you saved a little kid named Timmy from some gnolls, happily returning him to his mother and earning a beneficial ring. In mission 4, after destroying a cage, you release an undead Ghoul who is not hostile to you. His name is Timmy. Worst thing is that killing him will reward you with a Superiority Ring, which is far better than the ring you got from saving him.
Fortunately however, if you take a good look around Theramore Isle in the bonus campaign of Frozen Throne, you can find Timmy and his mother alive and well, so Timmy the Ghoul is likely a completely separate entity.
Arthas' The Undead status seems odd, since he was never shown actually dying. However in his last human appearance he was wandering off into the wilderness of Northrend with Frostmourne starting to drive him insane. He froze to (un)death there.