I think the main thing for me is the setting. I adore older settings, especially high fantasy, or something Ghibli-esque like Howl's Moving Castle and Castle in the Sky where it's not that technologically advanced but they have all these wacky contraptions and modes of transport and magic, it's the best. So if an RP is set in this kind of timeline, I'll always be tempted. I've never written sci-fi but slice-of-life sci-fi can be super cool.
So while these two ends of the spectrum interest me, I don't really have much enthusiasm for modern settings unless there's something that really stands out. I find it very difficult to conjure up inspiration for, say, a teenage Pokémon trainer in your typical journey-style RP -- and it's not that I don't enjoy those RPs (I sometimes check in to read the latest posts!) but I cannot for the life of me settle on a character or figure out what to do with them. Not sure why it happens so easily for me with other time periods.
And while I like sandbox, I think it's best when there is some kind of common goal or plot happening that players can participate in together!