Many people actually really enjoy playing slots. It's fun to win money, and it's fun to enjoy the dramatic moment before you know if you've won money. Some people get addicted to it, just like some people have gotten addicted to immersive PC games or really anything enjoyable. The distinction you're trying to draw is basically non-existent, except perhaps that social games and slots are simpler and revolve entirely around the unknown-reward mechanic.

That you can't perceive a distinction does not mean that it doesn't exist. People don't get addicted to everything fun in equal measure; some things are more compelling than others.

And seriously, go to Vegas and spend a couple of hours watching people play slots. Craps players are visibly having fun. The people who spend hours at the slot machines visibly aren't.

Craps, and to a lesser extent Blackjack, have some shared experiences between the players. In craps, many of the players will be betting the same, or at least very similar, way. Since there is only one dice roll for the entire table, there is a lot of shared excitement. With Blackjack, say the dealer is showing a 6, it's most likely that nobody on the table will bust, so if the dealer busts, everyone is excited. A slot machine, on the other hand, is an entirely solo experience.

I rarely play any floor games, but one time I was waiting for some friends to wake up and I wandered down to the casino to play some video poker. I ended up winning $1500 on a royal flush at a nickel machine. What am I supposed to do, start running around the casino in excitement? It was around 8:30 in the morning and the places was pretty quite. Had my friends been there, I'm sure we would have had some hi-fives or something like that, but if I'm just sitting there playing by myself, I'm not going to have any sort of substantial showcase of emotion.

I think you're correct in that a lot of people sit at the slot machines for hours, losing their paychecks in the process, and it's destructive for them. They may be looking miserable at the machines. The people playing craps who are in the process of losing their paychecks are equally as miserable, they're just showing different outward emotions. The addiction isn't any healthier because the person appears to be having fun.

I agree with this, but I'm not sure what your point is relative to the rest of the discussion. I'm pointing out that "fun" and "compelling" are distinct. You can engineer for either, or both. If you watch people playing slots, it's pretty obvious that most people who play slots for a long time are not having fun.

I guess I don't understand the difference.

I didn't once either. You may just not have much of an addictive component to your personality. But if you'd like to try an experiment, go regularly to play casual games at a site like Kongregate and focus on your feelings.

I discovered that some were fun and some were compelling. The two traits are definitely correlated, but there were plenty of games that were only one or the other. There were a number of compelling games that I fucking hated by the time I was done; they had stopped being fun long before they had stopped being compelling.

Did you find any that you kept seeing after you stopped playing, as with Tetris hallucinations?

Yes. If I play something intensely enough, especially if it's visually novel, I'll see it when I close my eyes before sleeping.