Resistances and damage vulnerabilities are a thing in TF2. They’re scattered here and there as pros and cons to using weapons. They often appear as passive pros and cons, but some weapons throw on resistances or vulnerabilities whenever you switch weapon. Whatever Valve does with them, I don’t really like them. Actually, I hate resistances.
First off though, damage vulnerabilities. Despite what you may think, vulnerabilities don’t really bother me. They’re easy for most players to understand, if you use, for example, the Candy Cane, you know that explosions are even more deadly than usual and you need to avoid them even more. The all-round damage vulnerability Marked For Death is also obvious – not only do you know you’re weak, but so does everyone else. All you have to do is avoid being seen as Marked for Death and you’ll be fine. It’s all so simple. And even if people don’t see that you have a damage vulnerability, it only matters to you, no one else.
Damage resistance on the other hand is a whole new kettle of fish, and it’s produced some of the most annoying weapons to fight against. The current winner is the Danger Shield, which has a bullet resistance built into it. Being a passive bullet resistance, it’s always there, so the shield is always visible, but the bullet resistance plays havoc with the whole meta-game Snipers have, of the better skilled Sniper winning. It changes the Sniper VS Sniper match up from “Who can aim faster and more accurately wins the fight!” to “Whoever wears the Danger Shield has a huge advantage and then afterwards if you’re both wearing a Danger Shield whoever charged first wins!” and it’s just… awkward.
Other awkward resistances are the Shields for Demoknights. The Chargin’ Targe for a while had COMPLETE AND UTTER IMMUNITY to afterburn, as well as a heavy fire resistance, making it incredibly hard for Pyros to fight against Demoknights. This match up ended up being “make sure you have a shotgun, spam airblast or you’re fucked!” which isn’t much fun for the Pyro.
Post-publishing edit: Somehow, I forgot about the Spycicle, and should probably mention that, thanks N03y3D33R. The Spycicle goes against all rules of clarity – you don’t know when the Spy has it available, you don’t know when it’s unavailable and you don’t know when the damn thing is active unless you’re constantly tracking and flaming the Spy. Combined with the Dead Ringer, you end up with someone with all sorts of damage vulnerabilities and you can barely keep track of what will and won’t kill him.
But with passive resistances, the big ones at least, you can see that they’re there. Mostly. The shields, both on Sniper and Demoman, are always visible no matter what weapon is equipped. Other resistances though are much less obvious, like the when-spun-up-and-under-50%-health damage resistances the Brass Beast and the Natascha have. You need to keep an eye on their health carefully and make sure you’re doing it right.
Speaking of doing it right, my favourite resistance is that of the Vaccinator. The Vaccinator has clear symbols showing that a class has a resistance to a specific damage type. But even those symbols aren’t always clear or visible. The shields themselves are very plain, and I’ve always wanted to have my own custom shields, one for each resistance type, to help players even more easily know what weapon type they need to switch to. Clarity is key, here in TF2.
Really, the same thing applies to all class-based bonuses, they should be clearly marked on a player, or weapons with passive abilities should be visible on a player at all times, even when not currently in use. But resistances often need on-the-fly readjustments, and not everyone has a Team Fortress 2 wiki memorised in their heads.