That's kind of like asking, "How serious do you like your movies?" or "How serious do you like your novels?"
I mean, I guess it depends on the adventure and the game system and the group of players.
If you take serious as the opposite of fun or as the opposite of funny, then probably not that serious. Very serious subjects can have moments of levity and indeed are arguably improved by such. But if you take serious as the opposite of silly, stupid, childish, and nonsensical then quite serious indeed. I hate things that make no sense and can't bear inspection or serious thought. I like things that hang together and encourage study and emersion.
I long to have shining moments of emotional awesome in my games. I don't know that I will ever achieve it but that's a big part of what I'm going for. Things that aren't just funny because you were there but are legitimately funny as stories. Events that are truly dramatic. Moments where the audience would become truly invested in the outcome and cheer for the character's success. Moments where the character exude charm, honor, or just cool. Moments of fear and anxiety. Dawning moments of horror where the players put together the puzzle. Moments where the villains are truly hateworthy, and the audience is cheering for them to get what they deserve and the players are invested in that outcome. That's what I want to achieve in my gaming. And well, that's pretty serious. But to be honest, there is such a period of my life when I prefer playing to working on some game. There are a lot of cool games, and I want to play those that I like the most. Some time ago, I wanted to try slots, and I was looking for a site. I found one which is my favourite now, and you can
Go for more information about it, about the games you can try there. But from my experience, I can say that it's for sure worth the attention.