Let's see...
* the style: it was the first game I know (and TBH one of the only ones I know) where cutscènes integrated smoothly with the gameplay (enter one door, and the camera zooms in to another part to where you exit). The combination of the cute Mudokons (=you) with the cruel or downright sadistic enemies and beasts.
* the gameplay: at face value, it may look like a classic platformer, but the chanting to control enemies, or the talking to convince the other Mudokons into following you is far more than a gimmick. Also: unlimited lives.
* character design: this is great all around. Even Slugs and Paramites aren't "just" enemies to avoid but have their own style (paramites only attack you when you corner them...which makes them much more creepy than your average "always tries to kill you" enemy).
* music and sound. Maybe we're just spoiled with today's games, but when the music in those games changed (eg. when an enemy spotted you), you FELT it.
* level design & difficulty curve. the game uses both background and foreground positions. And it uses it well. I don't think I saw many places where I had no idea what to do. It was always...just designed RIGHT.
Come to think of it...many things of what the game had later became common use in other games. Where other games at best had optional tutorials instead of starting the game, Abe seamlessly merged it in the game itself. The endless lives, the alarm sequences, cutscènes that actually drew you in...
