Can You Answer Yes?
Most of these aren't new, or surprising. But I'll go through the list, just in case:You have to love reading. Its like wanting to play a sport if you are totally unfit. Reading is a writer's workout.
Your story (especially if you're new) has to be unique. So, you need to know what else is out there - no point in recreating someone else's masterpiece. For example, are you planning on writing a series of novels about a scar-faced orphan and a magical school? Sorry. Too late.
You need to be prepared to put in the hours. Hours and hours and hours. Persistence is a very underrated virtue, but it's a requirement for a writer.
Training in writing isn't a necessity, but boy it helps. Firstly, it helps you sell yourself to agents, publishers, readers - credentials always look good. But secondly, and most importantly, training makes you a better writer. Training could be a certificate, a diploma, an MFA, a weekend away with other writers. Whatever it is - it's time spent, learning. And boy, there's a lot to learn.
Rejection is a given in this business. Unless you're really really lucky, and/or incredibly good and/or you've slept with someone, it's inevitable. It's horrible. It hurts. You need to be able to handle it. I'm still working on this...
Don't try and fit writing into your spare time. Something else always gets in there, too. Instead, make a certain time of the day 'My Writing Time' and be really really scary if ANYONE tries to interrupt you.
Are you comfortable with blogging/tumblering/instagraming whatever? Chances are, if you're reading this, you are. Congratulations! You will be a much more attractive option to an agent, and you'll have a ready audience for your novel. I had to learn the hard way and boy, was it hard. I wish I had started earlier. My mistake - but you learn from it. :)
But, most importantly:
Writing is like an addiction, but legal.