...you can apply almost any pre-existing interest to your career.
I've thought this for a while, but it was really crystallised for me this weekend. I had a friend to stay - when I met him a couple of years ago he was doing a literature-based PhD. Then recently he had one of those 'turns out I wanted to work in the library sector all along!' revelations you see so often on Library Routes and is now training to become an archivist. The amazing thing is, he's working at the British Library and is now working on an archive of stuff he was writing about for his PhD! He's literally following up the interests from his Doctorate, despite having completely changed career path. Archiving roles are great for that - I recently saw an ad for an archivist post working in a 'traditional folk dancing and music archive', or something like that; for someone, somewhere, who loves folk dancing and works in the library sector, that is literally the best job in the entire world...
I used to want to be a Careers Advisor, working in Higher Education. Various circumstances resulted in my working in a library, and I've found it sufficiently diverting that I no longer have my original ambition. But as it happens, I get to do all sorts of careers related stuff in my job - acting as a New Professionals Support Officer at CILIP, presenting on issues relating to our profession, creating a careers resource of sorts in Library Routes, etc. I also love writing, which I get to do here and in a bunch of publications too. It's amazing - I've ended up doing something I had no inclination of any kind to do until the day before I applied for my first library job, and it's all gone full circle and back to the things I loved originally.
If you're interested in marketing, editing, web 2.0, emerging technologies, computers, art, the law, music (and a whole host of other stuff, as well as the more traditional things like rare books or old manuscripts) - almost whatever it is you can apply that knowledge or passion to something in librarianship, whether it's part of your 9 to 5 job or your extra-curricular activities you do as part of being a professional.
This is a fantastic bonus, and one we should make more of when promoting the profession. How many other sectors can boast this much diversity? How many other jobs allow so many hobbies, passions, expertise or ambitions to be brought to the working table? Libraries FTW! :)