The psychology is very important. All the big players in this space employ people with experience designing actual Vegas slot machines, and the goal is to try t

The psychology is very important. All the big players in this space employ people with experience designing actual Vegas slot machines, and the goal is to try to replicate it as much as possible. Everything from the line patterns, to the way words and icons pop, to the arpeggiated audio cues, is taken straight from tried-and-true Vegas templates. For someone who has spent time in real casinos, these games will feel very familiar, and trigger the same emotions. The trick with online, free-to-play versions is to forcibly stop the player when they've run out of coins, and say "come back tomorrow to collect your small refill, or pay us $2.99 right now to keep the fun going". And then throw every pricy psychology trick in the book at them to try to get them to break the seal of that initial purchase (because then the rest come much easier).