Resistance equipment

Resistance equipment is the major source of the Resistance stat. It has next to no general usefulness, and is usually used solely in a raiding environment.

In the past, several bosses simply could not be beaten without sufficient resistance of the appropriate type. In extreme cases, especially with Blizzard's design strategy from around the time of Ahn'Qiraj through The Burning Crusade, this required building an entire set of gear for one or more "soak" tanks on a specific fight. However, Blizzard later acknowledged that such bottleneck encounters were more tedious than fun and largely abandoned this strategy.

While moderate levels of resistance may continue to be beneficial for certain fights, you no longer need much resistance equipment for end-game raiding.

Resistance Gear Quality
While leveling, resistance gear is usually not needed. There are some rare instances where it might be useful, but there hardly exists any low-level items with resistance. Prior to BC, the Molten Core was the first place which required high amounts of a resistance (Fire). Nearly every raid instance since then had one or more bosses which required some type of resistance - sometimes only on certain players (like the tank), sometimes on the whole raid.

Resistance equipment frequently has only two stats - a resistance and a high amount of stamina. In a wider sense, stamina can be interpreted as "resistance to all". In another sense, extra stamina while prolonging the time to die, does not mitigate damage as armor does. Using increased stamina in magic heavy fights, goes hand in hand with an increased healing requirement.

The problems with resistance gear are that it's rarely used, hard to find or manufacture, and wearing it increases survivability in a certain environment, but gimps most other character functions.

Some advantages of resistance gear are, the ability to ignore some encounter mechanics. During the air phase of the Sindragosa fight, hiding behind the ice tombs becomes less important with enough frost resistance, allowing more time to dps and heal, leading to more total damage or total healing. In some 10 player encounters, enough raid resistance will allow using 2 healers instead of 3, perhaps increasing raid dps. Resistance gear can lead to greater raid flexibility when a less that optimum mix of classes are in a raid.

This tradeoff, survivability vs. efficiency, always needs thorough consideration when choosing resistance gear. Use the best items available. Low-quality gear (like the green random BoE drops) is not really useful. It's much better to wear just one blue (or purple) item instead of a collection of greens. A similar example are warrior tanks using mail or leather resistance items instead of plate. Even in the most resistance intensive fights there is almost always some physical damage, and tanks should not nerf their physical mitigation too greatly. If there's no other choice such a measure might do the job, but the instances are designed such that good gear is available when a certain resistance is needed.

Farming points (like Justice Points) or materials (e.g. ) for appropriate resistance gear may still be an unavoidable part of raid progression.

Resistance & Instances
Although instances do not scale and thus become easier with time, the following information applies to when they were originally released.

5 Man

 * Fire Resistance was helpful for tanking Nazan in heroic Hellfire Ramparts
 * Fire Resistance was helpful for all group members at the Nethermancer Sepethrea in Mechanar.
 * Fire Resistance was helpful for all group members at the Wrath-Scryer Soccothrates in Arcatraz.
 * Shadow Resistance was helpful on the tank for heroic Omor the Unscarred in Hellfire Ramparts.
 * Shadow Resistance was helpful on the tank for heroic Zereketh the Unbound in Arcatraz.
 * Shadow Resistance was helpful on the tank for heroic Pandemonius in Mana-Tombs.
 * Arcane Resistance was helpful on the tank for Vexallus in Magisters' Terrace.

Classic raids
Molten Core
 * Fire resistance was useful here throughout, which is why many Tier 1 pieces and items from around this period have fire resist on them. In particularly, FR was the key to surviving the Ragnaros fight.

Blackwing Lair
 * Nefarian notably required players to have an to resist his fatal Shadow Flame ability.

Temple of Ahn'Qiraj
 * Princess Huhuran required 15 melee DPS with high nature resistance ("NR soaks") to mitigate her poison bolts at 30% health.

Naxxramas (original)
 * The level 60 version of Naxxramas involved nature, frost, shadow, and fire resistance at various points. Most notably, Frozen Runes could be found on the walls of the boss chambers and could be either used to craft Argent Dawn resistance items or consumed in a manner similar to a resist potion.

BC raids
Karazhan
 * The Curator's fight involves large amounts of arcane damage. However, the Soulcloth set soon lost its usefulness between nerfs (e.g. the adds spawning less frequently as of Patch 2.1.0) and higher-level gear.

Gruul's Lair
 * Nature resistance was helpful but not strictly necessary for the tanks of Kiggler the Crazed during High King Maulgar fight.

Serpentshrine Cavern
 * Hydross the Unstable in Serpentshrine Cavern required one maxed out frost resistance tank and one maxed nature resistance tank.
 * Leotheras the Blind in Serpentshrine Cavern required a tank or warlock in high fire resistance gear for tanking the demon phase.

Tempest Keep
 * Fire resistance can be useful for the off-tanks in the Al'ar fight in the The Eye of Tempest Keep.
 * High Astromancer Solarian originally required high amounts of arcane resistance, which was the primary use for the Soulcloth set. However, after she was nerfed and turned into a Baron Geddon-style fight, most people simply wore their Violet Badges.

Mount Hyjal
 * Several bosses in Mount Hyjal (Kaz'rogal in particular) are much easier with some shadow resistance spread over the raid. and  sufficed for the most part.

Black Temple
 * Mother Shahraz in Black Temple required the whole raid to be kitted in maximum shadow resist gear.
 * On several other bosses in the Black Temple, some amount of shadow resistance was useful too.
 * Illidan Stormrage required a warlock with capped shadow resistance.

Zul'Aman
 * Some fire resistance (mainly for the adds tank) was useful (but not required) for Jan'alai in Zul'Aman.
 * Some shadow resistance was useful (but not required) for Hex Lord Malacrass in Zul'Aman.

Later raids
Naxxramas
 * Sapphiron requires the whole raid to have frost resist when first gearing up. As gear gets better, a paladin aura or shaman totem should suffice. There is also an achievement for no one in the raid having more than 100 frost resistance.

Ulduar
 * Hodir requires the whole raid to have frost resist when at the gear level of this instance. As gear gets better, a paladin aura or shaman totem should suffice.

Icecrown Citadel
 * Although some frost resistance can be useful for tanking Sindragosa, it is not a major concern.

Resistance & PvP
In theory, resistance could be quite useful in PvP. Unfortunately, most experienced PvP opponents wear large amounts of Spell penetration gear, which negates resistance. Therefore it's usually not a good idea to sacrifice performance in order to wear large amounts of resistance gear in PvP. You are prohibited from changing out gear in an arena specifically so people will not, say, change to a Shadow Resist set once they find they are against a warlock and shadow priest.

The primary way to weather damage in PvP is by stacking stamina and resilience. These stats have the benefit of making you resistant to physical and magical attacks.