* Computer science fundamentals
To know the landscape and limitations thereof surrounding a product being developed. These skills can be learned formally through school or informally by continual pursuit, as many hackers find themselves doing who come from backgrounds other than hardcore computer science.
* Communication
With stakeholders, the business folk that green-light and also those who manage projects. This one is two part since it takes a good measure of both knowing how to cut through the red tape and get the jobs done, and also how to effectively communicate with them in the language they understand.
* Programming and devops
You need to know common idioms, a common framework or three, and the way web applications are deployed and hosted. If you have programmed a ton of projects over the years you will have seen firsthand enough of the wrong ways of doing things to spot it a mile away and be able to know when something should be refactored, rewritten, or not, and be able to review others code for the same.
I'm deliberately leaving off "people management skills" because I think #3 covers that -- managing the technical output of people is the most important part for managing technical subordinates, which you will likely be at least partially responsible for as a senior dev, even if it is for developers in a mostly flat org.