I can't speak for most people, but from playing it at a friend's place I can say that I find it just plain bland and boring.

Actually, the entire "battle royale" thing is pretty much lost on me. The whole experience just makes me wish someone would just show up and shoot me already so I can stop playing this sluggish, yawn inducing game-mode.

Honestly from what I've seen most of the hate just seems to be a case of "bandwagon hate" with borderline hipster like "it's popular so I hate it" (especially given it's popular with kids). Like honestly I rarely see people whose reasons for disliking Fortnite is "I don't really care for the gameplay" or "it's just not very fun to me," it's often just people hating it because it's a popular thing (the same shit happened with Minecraft years ago too).