I've been wondering: How much would full plate armour protect against the kinds of magical attacks we see in fantasy (particularly RPG systems)?
Other armour systems may also be interesting to consider.

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.

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

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.

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.