YMMV / Thaumcraft

• Contested Sequel: Thaumcraft 6 is considered by many to be a downgrade from the much-beloved Thaumcraft 4 thanks to its replacing the well-received node system with a non-interactive system based on vis per chunk, having less content than earlier versions, and replacing a system of progression with the wands with a singular gauntlet that cannot be upgraded.
• Demonic Spiders:
  • Wisps, a mob spawned from flux, and the most common thing spawned from flux. they fly and shoot bolts of energy at you, unarmored these bolts shave off 3 whole hearts of health and even with iron armor they still do a full heart of damage. they are also incredibly hard to hit unless you have amazing accuracy with the bow. They also like to attack any golem servants and, due to their high damage output, make short work of them. However, if you can find the node that they spawn from and encase it in any building material, they become much more manageable. When they cannot fly out of the reach of your weapons, they are very easy targets.
    • That said, many players in Thaumcraft 4 will seek them out for a good reason: They drop Etheral Essences, one of the only sources of the Auram aspect (which is used in a couple of important crafts, like higher tier wand caps).
  • Crimson Cultists come in two varieties: The Crimson Knight and the Crimson Cleric. The former is essentially a stronger zombie, but the latter is this trope, being able to fire semi-homing balls of lightning that deal good damage, or firing shotgun blasts of fireballs that can burn you and your surroundings. To make matters worse, in Thaumcraft 6, they may randomly just appear out of a portal while you're out and about, making them able to screw you over before you potentially have any gear.
• Hilarious in Hindsight: Ethereal Platforms from the Thaumic Tinkerer add-on serve as Directionally Solid Platforms that the player can jump on and fall through. Minecraft later introduced the Scaffolding block in the 2019-released 1.14 update, which does exactly that.
• Memetic Mutation: Flux Rifts from Thaumcraft 6 are rather memetic on the r/feedthebeast subreddit thanks to people's reactions to them, as well as their non-standard design compared to the rest of Minecraft.
• Nightmare Fuel: Whenever you delve into forbidden knowledge, it slowly inflicts Sanity Slippage onto your player character, resulting in some nasty side effects when you delve too far. Including cryptic messages appearing randomly, to some rather nasty creatures like Eldritch Guardians and Mind Spiders randomly appearing out of nowhere, and after a certain point in Eldritch research, things get... Odd.
• Scrappy Mechanic:
  • Warp, at least the permanent variety. It can be very annoying if you have a lot of it, and it's with you for the rest of the game, which can be quite long considering what game this is a mod for. You can temporarily (around 20 minutes at a time) get rid of it, but doing so requires non-renewable materials, and not the kind that's found everywhere like dirt. It's been criticized because despite this, it's impossible to access a good chunk of the mod's content without acquiring some—and for some, you need quite a lot of it.
    
    In later versions of Thaumcraft 4 it has been remedied - no matter how high your warp is, warp events will slowly become less frequent and less intense as time passes, eventually ceasing altogether. But if you acquire just a tiny bit of warp again, they will return in force.
  • The Thaumic Tinkerer add-on adds, among other things, an entire selection of extremely-powerful items made using Ichor. However, in order to unlock these, you need to perform all of the other research. Since this includes forbidden knowledge, you'll get as much permanent warp as possible.
• Self-Imposed Challenge:
  • Some modpacks, such as Blightfall and The Hubris of Magic, spawn the player in a world that is completely tainted, with the challenge that the player needs to settle in the Tainted Lands.