A lot of it is just encountering new challenges you haven't faced before. Learning to scale an application was certainly one for me. Another was writing a web

A lot of it is just encountering new challenges you haven't faced before. Learning to scale an application was certainly one for me. Another was writing a webapp where accessing the primary datastore was a 50ms round trip. Another was writing warehouse software that had to interface with hardware. And another was building a somewhat complex multi-system application that ran an ecommerce business.