Either great story or awesome gameplay (infinite, if possible, like The binding of Isaac).
I never look at rewievs, though, I just pick a game and if I deem it good then I try it. Rarely miss. And even if I miss, then I, most likely, play it years later, like Osu! here, I installed it in 2013 and I didnt like it, but here I am, 5 years later, remembered about it (a week ago) and can't stop playing it since =)