The most important thing.
Everyone who starts a magazine has definite passion in their respective genre and have certain idea about their audience.
The one thing they wish they know, sometime after starting out, is that striking a balance between the content that users like to read and what investors (ads) like the viewers to read, isn't always easy.
To get ad investors, you have to have audience and to sustain the audience, you need to have the investors' money.
Think it through.
I wish I had known that Steve Jobs would die early and any vision he had for the iPad and the re-birth of magazine publishing would die with him. It is an amazing device but iTunes sucks as a platform. Plus, as long as Medium lacks paywalls (or something similar) and lacks an embedded payment mechanism, there is no good solution.
But if you want to cry over spilt milk, schedule a few minutes and we can sit on the ground and share sad tales. {grin}