Software Engineering is unique in that it offers a gradual path from a hobbyist to a professional. Many professions do not. For example, there's a world of difference between building a dog house vs. a sky-scraper, or giving first aid vs. performing open-heart surgery. To be an authorized civil engineer or doctor, who offers products and services to the public, there are rigorous standards that must be met. It's not just that 10 years of hobbyist experience of building dog houses won't give you the necessary professional experience to design a sky-scraper, it's that federal law won't even allow you to authorize the blue-prints. For example, if memory serves me right, to authorize civil engineering blue prints, you need to first go to an accredited university, then pass a huge "Engineering in Training" exam, then work under the guidance of a professional engineer for 5 years, then pass a second exam. Then, and only then, are you legally even allowed to approve a blue-print. There is not a gradual path, but rather one with steep cliffs.
Software Engineering is totally different. There is a gradual path. You could start doing hobbyist web sites on your personal laptop. You can then continually learn more doing harder projects (perhaps for non-profits, or just for fun). All the while, you can read the same books, go to the same conferences, and use most of the same tools that the professionals use. You could then enter the lower rung of the professional world by getting a low-paying contractor job on a short assignment, publishing in an online journal, or contributing to a popular open-source project. From there, you could jump to larger teams, or do harder projects, and the rest is history.
This potential gradual path from hobbyist to professional has pros and cons. It offers great opportunity, because almost anyone who has the mental ability, passion, and a computer can enter the profession. However, the con is that many developers (doing paid, real-world, work), still treat it like a care-free hobby, as opposed to professional work.
What separates a hobbyist from professional? Scott McChonnell's good book, Professional Software Development, offers a lot of good ideas. I think some important traits of a professional include:
Having public process that works for a team of developers - i.e. a team build server, team source control, etc...
Writing code that works on other people's machines.
Writing code that scales, is secure, and maintainable by other people
Producing deterministic results
Providing schedules and estimates
Using industry best practices and design patterns, not just your own personal bag of tricks
Referencing official standards, not just "I heard somewhere on some blog"
Of course there's more, but this is a start. The good news is that even as a hobbyist transitions to a professional, they can still pick up things like these. It makes me appreciate Software Engineering as a truly wonderful field.