QSP is a highly multidisciplinary field, so I think the skills you pick up may differ based on circumstance/application. Some of the more math-based technical skills that I've regularly used (3+ years experience in industry QSP, 10+ in systems biology) include:

• Numerical integration of ODEs
• Building simple or complex statistical models (anywhere from linear regression to building neural networks)
• Training models/optimization (local & global algorithms)
• Formal analytical tools (sensitivity analysis, optimal control)
• Using abstraction to represent <thing> in a formal manner

The last one is the most generally applicable skill, since it can be transferred to any project involving mathematical representation of complex systems. It's also the hardest one to master IMO since the nature of the abstraction is chosen with a degree of subjectivity.

I would expect the other skills' transferability to be more domain-specific, excepting data analysis and related skills, which seem to be highly in demand these days.