I am pretty sure your employer hired you for serving a specific purpose, unless you are hired for the man-friday position. Why don't you go to the hiring manager and start a discussion with him/her, saying something like "I enjoy having the freedom of picking and choosing my tasks in my such-and-such years/months of employment with you, but I want to help our organization be more effective, by undertaking assignments which are crucial to the success of the team. What do you think I can start working on?"
Every professional organization has goals. Goals get divided unto smaller goals and the division goes down to the simplest of tasks to accomplish the goal. It looks like you are stuck at the bottom of that totem pole. You may want to broaden your point of view and instead of focusing of individual tasks, you may try to see the bigger picture and how those tasks you perform, come together for a greater purpose. It will give you the insight about each task in the project plan as well as planning/managing the whole project. This can easily catapult your career into a project management position with the technical expertise about the job you are performing. Once you have few of such assignments and successful completions under your belt, you can write your own ticket, without worrying your professional development not being as fast as you hoped it would be. Mainly because you will not have time to think about such minor details once you get into the big-picture world and there is a lot to learn that way.