Here are some random, stream-of-consciousness observations. Take 'em or leave 'em.
1. The fund of knowledge in Rad Onc is VAST. Seriously. Ridiculously over-the-top. Of course, this is something I had an appreciation for even before I started. This is one reason that academics is appealing, you get a sub-site or two and became a super-expert. Once I graduate, it is inevitable that less used areas of clinical knowledge will begin to deteriorate. The price you pay to maintain them is CME.
2. I learned more about the economics of Rad Onc during my interviews and with my new practice than during my entire residency. It's complex and forever a moving target. I think earning a straight salary, in some ways, frees you from some of the ethical murkiness that goes on with reimbursement. However, when you collect revenue (especially technical fees) you feel that nobody else is financially benefiting from your own hard work.
3. Rad Onc program rankings are flawed, in part, because of regional preference. Applicants who train in one area of the country x 4 years are likely to put down some roots, begin raising a family, and easily obtain local job prospects. Thus, it does not make sense to judge graduating residents in the Midwest on their ability to gain jobs on the coasts - they likely neither wanted nor tried to go there. Exceptions exist of course, but this is a general trend I've noticed.
4. If you want an academic job, you really should have a minimum of four publications (one for each year). If you go for quality rather than quantity, this may hurt you because it seems that most people are interested in numbers. I think, at most places, one paper in Nature won't go as far as four papers in the Red Journal, for instance. Physician-scientist jobs are a bit of an exception.
5. Print is dead. I know all of us like to tote around Rad Onc textbooks, handbooks, and guides because it "feels" good and there's nothing like the tactile feedback from turning pages. However, at the end of four years you will be stuck with out-of-date paperweights. Use PDFs and start backing up your data regularly. There are, of course, some "classic" books that will never cease to be relevant.
6. Rad Onc is a small, "gossipy" field. If you are an a-hole, it will go around believe me. It surprises me at how many callous people I see who utterly lack insight into how they treat others. Karma's a bitch.
7. Always keep your eyes and ears open for opportunities. Many times you will be shut out of something you try desperately to attain while a "golden ticket" you weren't expecting falls into your lap. If you fixate on the unobtainable thing, you may miss the "golden ticket." Chance favors the prepared mind.
8. If you are married or have a SO +/- kids, the happiness of your family is paramount. Don't go somewhere for a "perfect" job opportunity when your family objects. It won't remain "perfect" for long.
9. Don't be a radoncanon. People in this field tend to be nice, well-adjusted types who are not prone to vindictive rampages based on witty internet postings. Also, connections are vital and you can potentially make MANY on this forum. I can't tell you how many dozens of forum members I know personally and I've met in real life. These are great folks and you can learn from them just as you can help others. Don't be a lurker, give back.
