The single most important thing about writing an event is making people care. Regardless of how well written it is or interesting it may be by the advanced description, no one with the exception of people who either hate or love you will care unless the basic and medium descriptions give them reason to.
It's debatable whether or not the medium or basic description is more important, but together they serve as the basis for creating event RP. The basic description can be compared to a hook with which you can catch the interest of other players. It should ideally be something they would come across without much effort such as something popping up on the news or something flashy changing around town such as a new business or act at the Bronze. Basically the less subtle the description, the better the event. I'd take an event with flying monkeys vandalizing main street over your character taking a yoga class any and every day.
The second most comon problem I see with events is that after researching them, I don't actually know anything more than I started with or, ocassionally because the basic description was just so terrible, I know about as much as I should have known anyhow. By this point people should not only want to, but also be able to put together a group of people for a reasonably fun research scene. I think a good test whether or not your medium description is inadequate is to ask yourself if someone could put together a successful and entertaining research attempt without any further input from you. I rarely, if ever, feel like people give enough information about what their event actually involves by this stage.
Anyhow, take it as you will. I think it's sound advice. My events aren't perfect either, but I think if everyone keeps those two points in mind, a lot of events should be more interesting. I also wrote a guide a while ago that touches on this.