Adaptation Distillation: Plenty. Nemezish was blind in the original FRPG - no sign of her blindness in the game. Gnarog wasn't even a human, but a half-elf half-orc. Many of the interactions between the character that took place on the original.
Adaptation Displacement: May or may not happen in the future. The original FRPG was Hungarian-exclusive, and was known only to at most 10 Hungarian persons. The game is available in both Hungarian and English, and may become known by more than just 10 people in the future...
Author Avatar: All of the main characters (Gnarog, Nemezish, Ildor, Dermaglen, Kawazoe) are avatars of their original creators, most importantly, Gnarog being the avatar of Zsolt Tóth. Kawazoe, the god-like character is the avatar of the original creator of Alternate World.
Demoted to Extra: Ildor is a gameplay-wise strong and plot-wise important character in the Spirit World Arc. The Artograch Arc and onward, he becomes gameplay-wise useless (overshadowed by character with powerful spells, like Nemezish, Dermaglen and Stephanus) and plot-wise "silent". He doesn't really play any real role - he's just there.
What Could Have Been: The Artograch Arc happens to have a day-night circle and a full world-map - much more of Artograch was intended to be visitable. So why are the only visitable places Grandfolk and Magnus Nex Urbs? a) Because of the creator's laziness; b) Because they are plot-central locations, while every other part of Artograch is irrelevant to the plot.
The Terran Arc was intended to be longer and to involve lots of side-quests. Instead, all we get is a visit to an Israeli airport that takes us to North Korea, where we fight the final boss Kim Jong-Il.
Speaking of side quests... Side quests were planned for the Artograch Arc too, but no such thing became reality.
Some planned sidequests would have involved travelling back and forth between Artograch and Earth.
A remake was considered too, but got scrapped before even more serious considerations. However, after this article got written... there may be a chance that there will be a remake with more content, a more streamlined story, more characters, maybe even voice acting!