Link Your Stylesheets
Along with your default styles and your additional styles for any plugins you are using, strongly consider using a CSS reset or as an alternative, something like Normalize.css. I personally use Normalize, but there are plenty of resets that will do a great job too. These will make sure that your code stays standard-compliant and working in as many browsers as possible. Nothing is more annoying than your site having five different looks for five different browsers because they're all reading your styles differently.
Add Your Fonts
Adding a link to the fonts you're using will make sure that even if the viewer of your site doesn't have the fonts downloaded, it will still display like you intend.
There are a few different ways you could do this. You could add your own fonts via a folder in your website and serve those to the user with something like Typekit. Adding your own is great if you have your own custom font you wish to use. If you don't have your own, think about using a service like @font-face or Google Web Fonts, which is shown below.
Add Your Scripts To The Bottom Of Your HTML
Titles and Meta
Be good to your SEO. Make sure to do the minimum and add a title to each page and a meta description.
How To Make Sure You Don't Miss Anything
If you're unsure what you need to add to make sure your site works to its full potential, consider using a boilerplate. Boilerplates are basically extremely light frameworks that will set you up with a starting point with all the details written for you. A good way to look at it is it's a tool that does all the things you would do anyway.