As a dominant the most important thing I want to see in a sub is good communication. You need to be able to say what you'd like, and speak up when things are wrong. It's the only way I can adjust and make things better for you.

I'd also like to see a sub put a bit of effort into the things they want to do and the things they want their Dom to hold them accountable for. Some things in a scene require practice or prep work. And if there's anything you want to work on about yourself, it's nice if the Dom isn't chasing you up constantly to get things done.