I guess it could be:
- Writing: the ability to write great actionable content and copy that sells
- SEO: generating organic traffic to your website gives the best ROI of all the traffic sources
- Digital Advertising: understanding PPC and the ability to lower CPA running successful campaigns after successful campaigns
- Technical: whether it's writing code or doing designing work to make sure you website looks/runs perfectly
- Networking: involving social media optimization and influencer outreach as a way of getting referenced as an industry expert
- Creativity: the ability to come up with the 'WOW!' factor in marketing campaigns that make people click every single time
- Data Analysis: understanding data of your website and what you can do to optimize it, whether it's Google Analytics of PPC optimization
And many many more can be added to the list.
I personally believe the one trait to be successful is curiosity. Curiosity will lead you to ask "why are people not subscribing more" and "if I change something will it increase conversions?".
That leads to testing, fixing things that don't work, and doubling down on things that do.
A very close second would be adaptability. The beauty of the Internet Marketing world is that two days are never the same and as problems change so do the potential solutions to tackle that. Thus the right mindset to always be adapting is essential.
What do you think is the most important skill?