These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Demonic Spiders: Units with the assasination trait, which gives them a chance to One-Hit Kill anything that isn't a titan or a building, including your hero.
Minotaur Kings, they can negate your defence so even the highest level/naturally powerful units can die in a few hits (even dragons and titans), inflict fear in lesser units, can attack any kind of unit, can heal their hp and cleanse poison or disease by eating animals (which the minotaurs can produce in 3 seconds for only 1 gold).
Dwarf Lords when they are deployed at the beginning of the game: They cut elemental damage in half, take minimal damage from ranged attacks, inflict extra damage on buildings (which pretty much makes them tower destroyers) and still can get all of the armour updates, which, these being dwarfs, boost their already formidable resistances even further.
Any Dark Dwarf golem becomes this when you don't have magical or electric damage, since they are all highly resistant to melee, and can attack both ground and air units. The Iron golem takes it further by being able to hit all enemies around it, and cranking out Firebombs for free, while the Bronze Golem epitomizes it by doing away with their speed restriction. Oh, and they also get massive armor and damage boosts, like regular Dwarves do.
Empire's elephants, they can be deployed at the beginning of the game and inflict extra damage on "small" units (which is most of the units you are getting at the beginning of the battle, including your hero).
Knight Lords can become this, they are naturally powerful, and can get all of the updates cavalry can get, but the knight's updates allow for faster exp, so not being able to take out a Knight Lord quickly might end with an overleveled knight running through your base.
Shadows. They ain't a big deal normally, just rather flimsy ghosts with cold damage that take a level in badassery when it's night. However, they do have the normally only Goddamned Bats-level ability to drain experience and take it for themselves when getting the killing blow on someone... where the nasty, nasty, NASTY surprise comes in is when a whole crapton of them gang up on your hero, or another, and kill it: Their experience counts too, all five-thousand-or-so points of it. Congratulations: You now have an angry, army-murdering, tower-shattering ghost from hell about to rampage on you and your base! Oh, and both Undead and Dark Dwarves can rally them out relatively early on.
The Plaguelords in III are incredibly overpowered, have access to cheap hordes of units and very powerful area-of-effect monsters. Their only downside is a lack of ranged units, and that they run a little out of wind in the very late game, which considering the amount of races that need to pass through a weaker early-and-mid, tends to be more of an advantage if played well.
In the first two games, a character who was created for the campaign mode could not take part in skirmishes (and receive experience for it) until they finished said mode and exported their user data. In the third game, these two modes were integrated, meaning skirmish matches could be played independently using the same campaign hero. A savvy player could set up matches against ridiculously-hard AI opponents who had little to no resources, then reap hundreds of XP points in a short time. By combining this with a few upgrade points pumped into the the Knight Lord skill for the Knight class, a player could conceivably start a campaign mode tens of levels higher than the initial difficulty, and (by spending all starting XP on Knights) have a posse of bodyguards who will ravage anything and everything in their way. The campaign becomes trivial as a result of this, even on the highest difficulty.
Most Annoying Sound: The warning sound (a steadily-rising heartbeat) as the player nears death.