We’ve still yet to put our developers, both back-end and front-end (I’m both really), into the process early, but with constant determination the project managers are learning to bring us in earlier and earlier.

As you stated and which can not be stressed enough, learning the entire stack is the single most important thing. There is no doubt that should you ever need to move to another company or position that this will help you along.

The only addition I would add is, though this ties into #1, is to have great people skills. Because you’re the ‘go-to’ person for a lot of things, you need to learn how to not only manage yourself and your projects, but people too. Keeping the Project Managers of Doom happy while keeping yourself less stressed is the only thing that has saved my sanity through the years.