The key to a usable product is knowing your user. What are their goals & needs? Once I know that, I can focus on the user flow. What steps are necessary to achieve your business goals while satisfying your user.
A clean interface gets out of the way and doesn't distract. Nothing flashy or over-designed. A good design should represent you and help your users. Harmony and balance are powerful tools to achieve that.
Creating a design that works on all screen sizes requires more than just creating a separate version for a phone, tablet and a desktop screen. It's all about creating a style and behavior which works consistently across all devices and sizes. That's why I consider the "act" of Responsive Design not as a single task, but the bridge betweend Design & Frontend.
For a responsive design to work well in all environments, it's important to know your ways around the frontend. Having a background in frontend, I live and breathe responsive design, always pushing the boundaries of the frontends and interfaces I design and build.
Design, Frontend-Development AND WordPress? You must think I'm a Unicorn. I'm not, personally I just dabble. But through my company required.ch, we can offer theme, plugin and custom WordPress development.
Having worked a couple of years in an email marketing agency, I also know my way around html emails. Although this isn't my main focus anymore, I still design & develop the occasional responsive email campaign.