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: Both literally the poisonous spiders and the zombies later on in the game, the spiders are agile and hard to hit and if they bite you, you get poisoned, zombies can also puke poison quite a distance at you. If your poisoned there's no way of curing it apart from antidote and there's really very little in the game. So you'll most likely have to keep healing as you try and find an antidote.
Draco in Leather Pants: Xana, ironically...or not. Just go onto any YouTube video featuring a scene with her and read the comments. You will begin to wonder if some people understand what "Succubus" means.
Evil Is Sexy: Very obviously deliberate in the case of Xana, who is only a voice in your head for much of the game, yet most people agree that she still manages to leave her Incorruptible Pure Pureness rival for your affections in the dust.
Fridge Brilliance: The final fight with Arantir has him summoning a Spectral Dragon to attack you. In HoMM5, Arantir has a default ability called Avatar of Death, which summons one highly powerful Spectral Dragon onto the battlefield.
Game Breaker: The kick attack. It does minimal damage, but it cannot be blocked and always does knockback. Abusing the kick to push enemies into the astonishing number of fires, chasms, and conveniently-placed spike racks can make several sections of the game very, very easy. After patches it's mildly balanced out by being relatively tiring, making it harder to spam.
There's a reason 1up.com/Computer Gaming World/Games for Windows dubbed it "The Adventures of Sir Kicksalot Deathboot in the Land of the Conspicuously Placed Spike Racks".
Standing on knocked-down enemies will prevent them from getting up again, but doesn't stop you unleashing finishing attacks on the fallen enemy. The Duel Boss becomes much easier knowing this.
Late in the game, there's the Lightning Shield. It's a little tough to find without a guide, but it has infinite durability and stuns enemies whenever you block an attack with it, leaving them open for an instant kill.
The Sanctuary spell for mage characters. It gives you complete invincibility for decently long time (compared to other FPS games), uses up only about one third of maxed out mana bar and has a cooldown period that's shorter than its duration. Have you been stacking on mana-restoring items? Enjoy god mode.
Enemies who are on fire lose health at a tremendous rate and frequently cannot attack while in the midst of their pain animation. There are oil jars scattered around the levels that can be broken on the ground or an enemy and there are a variety of weapons and spells that can set said pools on fire.
Porting Disaster: The console version, DMM&M: Elements, is considered such (a widely cited reason being that the customization is limited by classes).
Scrappy Level: Chapter 6's crypts. Poison is insanely deadly and this level is filled with poison spewing zombies that are hard to kill. On top of that, it's a confusing puzzle fest that is hard to navigate.
The first spider lair qualifies for some as well. Spiders inject you with a venom which can kill you in seconds, they can soak up large amounts of damage (although fire of any type does massive damage against them), and there's only the opportunity to grab one or two antidote potions. Many game guides recommend just running through the level at full speed and healing up at the end.
The Untwist: Sareth is identified as a orphan in the manual right next to the bit that the Dark Messiah is a half-demon child. Is the "twist" really that predictable? Yes, yes it is.
They Wasted a Perfectly Good Character: Arantir, despite his status as main antagonist, feels very underutilized. This is particularly disappointing since he's pretty much the only character with any depth to him. Duncan gets it even worse. He has about three lines, yet still manages to be very sympathetic.
Arantir was eventually addressed in the Tribes of the East expansion for Heroes of Might and Magic V, which expanded on his Backstory and provided the reasons for his motivations.