Stone Degradation (soft stone)
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Splash-out and evaporation in areas without a lot of summer rains. So stone coping, mostly.
Possibly outer areas of waterfalls (where water splashes and evaporates).
In particular (as waterbear pointed out), limestone is the worst, though flagstone and really most stones have a variety of hardness so this varies by quarry (so, location).

Metal Corrosion
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Aluminum top rails for auto-covers
Aluminum header bars immersed for "vanishing" auto-covers
Aluminum or steel filters (rare these days as they are mostly plastic now)
Diving board base (and screws/bolts) from dripped water
Furniture (mostly screws) from dripped water
Inferior stainless steel or zinc-coated materials (such as screws in Intex pools)
Screws, rails, pool cleaner parts (some are metal, such as threaded gears powering the wheels)
Copper heat exchangers (cupro-nickel and titanium are more resistant)
Galvanic corrosion at bonding wire where dissimilar metals meet (occurs anyway, but might be accelerated by positive voltage on bonding wire)

Not all of the above are equal in susceptibility. Problems with heaters, for example, are more rare.

Also note that metal corrosion is significantly exacerbated if there are stray voltages which can occur in some areas. The higher conductivity of the water just makes an existing problem occur more quickly.