Here are the three things which make a good Scrum developer:
1 - Engineering Practice
Mastery in Engineering Practice is very hard to achieve - but if you don't even have these as basics, everything you produce will most likely cause more problems than it helps: Technical debt is much harder to manage than avoiding.
Getting the basics down on these only takes a couple days - getting the practices on a value adding level takes a fairly long time.
Think, among others, of:
Emergent Design - The art of designing your code so that you are never maneuvered into a corner where you can't meet a reasonable customer request due to technical constraints.
Sustainable Test - Moving from simply writing unit or behaviour tests to the level of testing the right thing, the right way, to get meaningful feedback at minimal cost - while equally avoiding the pitfalls of flaky tests and constant test rework.
Clean Code - Applying SOLID principles "just enough" to end up with easy-to-understand, well-tested, robust code that does not rely on heavy documentation and is easy to work with.
Refactoring - Tweaking the code just at the right time to make it fit for the current purpose without breaking anything.
2 - Lean Thinking
Your horizon must extend far beyond the Software level, Keep Lean's "Optimize the Whole" in your head. You must grow your product, avoid waste, eliminate "work to do", simplify processes, keep customers happy and your organization profitable. At the same time, you don't want to get stressed - and to focus: Quite a feat. Can you properly apply these to your work?
Value - Discriminating "what is asked" from "what is useful".
Value Streams - Discerning the easiest way to get from customer need to customer satisfaction.
Flow - Slicing down the work, avoiding BUFDs and Big Bangs.
Pull - Leaving the tiresome treadmill of featuritis, moving towards sustainable pace without losing business opportunities.
Perfection - Never being content with "good enough", while at the same time avoiding the waste of "gold plating".
3 - Agile mindset
A key pillar of agility is the Inspect and Adapt mechanism. Regardless of how professional you are, you're probably not a prestidigitator. How well do you do in this arena?
We don't know - There are many things we don't know. Some things we know that we don't know, and other things we don't know that we don't know.
We must learn - We must continually strive to learn and incorporate our learning into our approach and behaviour. We must accept that yesterday's "right" may be today's "wrong".
We must collaborate - No one person can know or do it all. Only by collaboration can we attain the best possible outcome. This collaboration includes developers, business people and customers alike.
SummaryBeing a good Scrum developer requires much more than being a good programmer. The mindset change and learning required to grow into this role may require quite some time, but it's a road definitely worth taking.
Of course, understanding your delivery framework, such as in this case, Scrum, is also advised, but that's probably the easiest part to pick up.