I never got that far into Divine Divinity: Original Sin, but the thing that really impressed me and kept me playing as long as I did was its combat. More specifically, the level of interaction between spells, elemental effects, and the environment was fresh and engaging.
Electricity doing stupid damage to characters or enemies with the misfortune of standing in water has been done many times in video games, but Divinity: OS took it much further.
For example, setting a poison gas cloud or a pool of poison or oil on fire would go BOOM. Sometimes, like if you chucked fire at a vent continuously spewing gas, this would yield some glitchy and/or funny results.
Electrifying wet characters or surfaces could produce a stun effect. Characters moving over ice had a chance to stumble and fall prone.
All these interactions provided a great range of tactical possibilities and made combat quite exciting.
Likewise, Has-Been Heroes implements a number of elemental combinations.
Water spells can soak enemies and cause puddles to form. Electricity arcs between targets and deals double damage to anything that’s soaked.
Poisoned enemies, when set ablaze, take a burst of damage (which expends the poison). Puddles exposed to poison become poison puddles. Poison puddles exposed to fire have a chance to turn into lava.
Soaked enemies, when frozen, stay frozen longer. Soaked enemies exposed to wind become frozen.
I’m sure I’ve left some combinations out, but you get the idea.
I was just thinking – I don’t remember any kind of elemental synergies or interactions in D&D (3.5, at least), aside from maybe using magical fire to light up some oil or something of that nature. Maybe they wouldn’t work or would be difficult to balance. Still, they could add for some cool gameplay.
All I know is I’ve really enjoyed the elemental mechanics employed by these two video games and am hoping more games get similarly creative in the future.