I'm trying to decide if Elemental Affinity is a talent worth taking. Would standing on plain ground reduge my Geomancer skill costs, or would I need to be standing in oil/poison to achieve that effect? What are all the different surfaces that trigger it and how how can I deal with the negative effects they have, if any? How often do these surfaces appear in-game and how easy is it to create them?
I did not take this talent myself when I played through the game so I do not know how effective it is, but I can tell you about the surfaces.
Put simply, the elemental surfaces are a core part of the game.
They appear very frequently, and there are specific spells in each school that create these surfaces or conditions (sometimes as a side effect, sometimes as the primary effect of the spell). The game isn't shy about being "video-gamey" and placing breakable barrels/containers filled with the various elemental stuffs. Even if you are at a lack for spells and containers, you can loot or buy spell scrolls, which are quite common, to create the desired effect. As long as you put a little forethought into how you plan on using your abilities, you will have no problem securing a reliable way to produce the elemental surface you want.
However, most of them require you to be on a hazardous surface of some kind.
Frankly, not enough research has been done on this talent, which is incredibly surprising given how long the game has been out.
Geomancer skills require you to stand on ooze or poison clouds, which both poison you. I believe it also works with oil, which slows you. All of these things are highly flammable.
Hydrosophist skills require you to stand in water puddles, or for it to be raining which is easily doable with one of the starting skills. This is by far the safest school for the talent. You don't need to be near the action where the blood puddles are and you can create your own surfaces extremely easily with Rain.
Pyrokinetic skills require you to be standing on fiery ground -- looks like lit coals. It probably also works when you are personally on fire.
Aerotheurge skills require you to be standing in electrified water. This is a harsh one because you will be stunned. Leaving your mage stunned and vulnerable is not exactly the best strategy.
A small secret: Witchcraft skills, despite not being strictly elemental, will be reduced in cost if you stand on a puddle of blood. Good for melee hybrid characters, since physical weapons produce that the most reliably.
Others that I haven't experimented with, but seem like they should be effective for at least one element include steam clouds (result from using water spells on a fire effect). I couldn't find anything online about other effects either.
These things are still hazardous and can still harm you, but from my understanding there's a talent that negates damage from environmental effects specifically, and it's not that hard to get items with resistances specific to the things you plan to be standing in.
In conclusion, you will find yourself with no lack of opportunities to use Elemental Affinity. It seems like a fairly safe option to me as a build choice, because resistances are always useful anyway and so is being immune to environmental hazards. 1 AP reduction is something you can go without but it's also a quite helpful bonus.