A few things about the elements I have noticed so far.
Note that they might not be 100% accurate, please correct me if you have evidence they're not:
-There are no actual "astral" and "umbral" elements, but they are a combination of fire/wind/lightning and ice/water/earth respectively;
-Just like their FFXI's counterparts, debuffs are aligned to an element even though the game doesn't inform you of that. Silence is still wind-based, Slow earth-based and so on. This is suggested by the fact that elementals can cast the same debuffs they did in FFXI and that they seem to resist them pretty often;
-Partial and full resists on nukes (NOT debuffs) do not seem to be affected by elemental defense. You can fully land something like Scourge on a Skeleton, but it will do less damage than normal;
-While in FFXI, being "weak" to an element meant to have less chances of resisting it, in FFXIV a mob weak to a certain element actually takes extra damage from it. Cassiopeias take extra damage from Thunder and Skeletons from Banish to name a few;
-Elementals do not have perfect resistance against their own element and the one they're strong against like in FFXI. Slow can actually land on Earth Elementals, though not as easily;
-Allocating enough points in elemental affinities seems to have an impact on the resist rate and/or damage you take from that element (i.e. I had 80 wind resist and Aero II's landed for 0 on me pretty often; I had 15 fire resist and bomb-type mobs murdered me).
XI Wiki



