I generally would have no answer to this. I don’t buy/read other TTRPGs.
But after reading this:
I began to look forward to being a grandfather (no sign of it yet), just so I could “play” it.
Easy to understand system - which I grokked - incredibly well explained with evocative example after example after example which showed me how to do it and formed wonderful pictures of the experience in my mind.
The subject matter has to engage my inner child and it has to be really easy to use.
The things that fascinated my as a kid, largely from the world of comicbooks and animated shows, are still the things that excite me in a rolplaying game. In other media I can appreciate darker, more grown up themes as in Breaking Bad or the recent Chernobyl, and I enjoy a night at the theatre as much as the next guy. But when gaming the grown up goes a nap and the inner child comes out to play. Which is odd, because I only discovered roleplaying games as an adult. So for "GM shivers" I'm looking for something that gives me the same buzz as a new issue of the Fantastic Four did back in the day.
But I also a practical creature. I don't get excited by a game if I don't think I can actually pull it off; otherwise you are just setting yourself up for disappointment. And I don't mean pull it off under the ideal conditions, with the right players, under circumstances and when rested and well prepared. That never happens. I need games I can pull off on a rainy Wednesday evening, after work, when I have a headache and the pub is no noisy we can barely hear each other. Having learned this the hard way I now automatically discount any game that requires too much of the GM and players to work. Clear premise that gets everyone quickly onto the same page, rules that are easy to learn and use, no intricate setting lore to act as barrier to players.
The bad news is that these to criteria eliminate a lot of games, a many of the best selling ones.
The good news is that the two criteria often go together. A game designed to capture the simple joy of a Saturday morning action cartoon is likely to have simpler rules.