I don't normally swing into these threads, but I've got a history of creating amazing RPs and then hosting them poorly so I think I have some good answers for this question.
Being able to create a compelling universe and premise is definitely important, but that's only the first step. A good host needs to be able to run a plot both on the long term and the short term, as well as improvise a few things as needed. The hardest part of running a plot in my opinion is finding that perfect balance where you aren't railroading your players into the plot, but don't have them just running around like a bunch of uncoordinated headless chickens. It's also important to have the answers to any questions you may be asked, which means lots of worldbuilding behind the scenes, even if not all the information is publically available. One of the most important things, though, at least in my opinion, is coordination. A good host coordinates things with their players, their cohosts (if they have any), and themselves. If everyone's on the same page, the rest should come naturally.
That's the short version of what all I have to say on the topic, since I don't wanna spend all the time I'd need for the long version =w=. On that note, dedication and activity are also imperative to good hosting; without those, and RP will inevitably sink.