One thing that I have tried to improve and work on is my writing and punctuation because it makes you look more credible and professional to your site readers. I admit that it had been years since I had to worry about what my writing looked like, but blogging has changed that for me. I still have aways to go, but I can see I've made a lot of progress since my start.
I look back at some of my early posts and cringe because I see how amateur they look to me now that I've been around the Blogosphere for a minute. Your appearance means everything in the real world and online as well. You want people to have a good first impression of you and your blog if you plan on growing your readership. People don't like to waste their time and you have about 10 seconds or so to convince them to stay on your site.
You also must have quality content and it all ties together. I found a really good plugin for Wordpress.org that helps you with spelling and punctuation errors before you publish a post. It will not only tell you what is wrong, but offers suggestions to fix whatever is wrong. I love it so far and it has helped me to continue to improve! The plugin is named "After The Deadline" and you can get it by itself or as part of "Jetpack" which is a collection of plugins made by Wordpress.com and they are all free!