Formatting - Your post need not be a circus. If you are not good with formatting, then stick to a theme that is light and descent in colors and the post layout. If you are good with trying new stuff then use that skill to better express through colors, fonts and layouts. Avoid colors that are dark and fonts that are overtly bright.
Remember that content is the key to a successful blog, Quality scores over quantity in getting audience to your blog.
Broken links irritate visitors and can badly affect your ranking. Check and repair all such links on your blog.