Got a Real World Weapons or Armour Question? Mk XII

[Brother Oni]

My thoughts:
Fire: Should be good protection - the conductive steel spreads the heat out over a wider area, while the padding beneath spreads it over time. Also, a layer of sweat should be helpful. Just hope that the fire doesn't reach your face or your eyes and lungs will be in trouble. Also, watch out for the cloth padding catching fire, though there won't be much air for it.
Heat exhaustion may be an issue.

Very much depends on the fire attack. A sufficiently hot and condensed bolt of fire would just melt through the steel (and by extension the wearer). Likewise, a lower but more extended exposure to fire would just set the person on fire like a flamethrower.

As for the padding igniting, some of it is still exposed to the atmosphere, where there's plenty of air for it to burn in.

Frost: Similar to fire, but reduces the problems of overheating rather than increasing them.

Similar issues as to the fire attack - extreme condensed cold would supercool the steel, making it incredibly brittle, while longer lower intensity exposure would cause frost burns to skin in contact to metal, not to mention the possibly of hypothermia.

Lightning: You're wearing a Faraday cage, so it should be much less dangerous than normal. Incomplete sets of armour (eg without greaves) may prevent grounding, which makes it more dangerous.

On top of the other suggestions, you'd have to assume that your skin isn't in contact with the metal or the padding isn't sufficiently damp enough with your sweat to conduct.
That said, you don't need much current to pass through your upper torso to sufficiently disrupt your heartbeat.

Acid: Most of it will get stuck on the plates. Should stop personal damage, but the steel will be compromised. Again, hope to avoid face hits, because of the breathing holes and eye slot.

Ah, something I'm on a bit firmer ground with.

Depends on the acid although any acid worthy of the name will have a major affect if it hits the face and gets into the soft tissues. Regardless of type, a long duration spray would probably incapacitate the target at the very least, if not kill.

Concentrating on small 'bursts' of acid, lets take something nice and simple like concentrated hydrochloric acid.
Against steel plate, it won't do a thing - iron (let alone treated steel) simply isn't reactive enough for short term exposure to HCl to do anything; it's more likely to drip off before it starts corroding the plate.
Even soaking into aketon/gambeson isn't going to degrade the padding quickly and it'll take a few minutes for it to start burning skin.
The acid fumes however will have a similar effect to tear gas on the wearer though and will probably be very debilitating.

Something significantly nastier like hydrofluoric acid... there's not going to much left of the target (or the thrower if they're not careful).

Suppose we had a theoretical acid like the xenomorph's blood from the Alien franchise; it's going to eat through the armour quickly, with the fumes being significantly disabling.
However when Cpl Hicks was hit with the acid, his armour was designed to breakaway quickly, thus it limited the damage the blood did.
There won't be such an option with plate, which requires somebody else to help armour the wearer (normally a squire's job for a knight), so the acid will inflict more injury or even kill the target unless someone's there to get the affected pieces off.

Edit: Sorry, forgot to say that if you offered a bit more detail regarding the cold/fire attacks (a particular RPG system for example) then we can start giving more useful suggestions.