1. Code quality (mainly with a view to maintainability)
2. Unit test coverage (I don't even asked them to do that, the good ones know to provide them).
3. How the app uses dependencies (APIs, databases, caches), and if they support dependency injection.
4. Can the app scale up and down (i.e. can I scale horizontally by just adding another instance?).
5. How are they using 3rd party libs?
6. How well documented is the solution?
7. How easy is it to build and deploy?
8. How well structured is their database (PKs, FKs, indexes, no excessives joins etc.)?
9. Is their a clean separation of the main modules in the app? E.g. MVC, having a REST API for the data and de-coupled clients etc.
I could go on. At a senior level, I just assume an engineer can write nice code in whatever language, but also has a keen understanding of software architecture with a view to how their code impacts on scaling and performance.
As a candidate, if all I get asked at interviews is low-level algorithms, I move on.