Very well said and pretty spot on. Given the vote patterns on woot, I don't know how valuable this will be to winning derbies, but it's definitely stuff that anyone who wants to really push their work forward should think about. I don't know what I agree with most, but I definitely have to say that looking around is a great idea no matter who you are or what intention you have in entering. Being educated about what's out there not only keeps you from straddling the line of the rejector, but it gives you an idea of how to use a shirt as a canvas... you wouldn't learn guitar based on vocal lessons, and just as both of those applications are music, both shirt designing and drawing are both arts that can inform each other, but are not the same. Learn what works, try to figure out why, and try to put that into your own art.
Not only that, but shop your work around to these other places. If you submit your work to Threadless, you're going to get a totally different consumer base to tear your work apart. Learn from them. Take negative criticism just as seriously as positive, if not more... you learn more from hearing what someone finds wrong about your work than you ever will from brown-nosing. Don't think that because you're a woot artist, you can't shop work to other sites... not to sound like a broken record around here, but Artulo is one of very few wooters to start here and get printed elsewhere (Design By Humans, no less, which is about as un-woot as a site can be). To the same degree, understand what a site is looking for... you might be a titan at woot, but don't expect to be as well received elsewhere.
Which leads to my bread and butter point... be true to yourself. The average derby has about 300 entries, which means you have a 1% chance of placing no matter WHAT you enter. Even if you're of the belief that only 30 entries per derby even have a chance to win, you're still at a 10% chance, which is still terrible odds. You're effectively designing for a portfolio with a slight chance of profit. Create a portfolio that tells future clients what you are artistically, not what you think others want you to be. Create a portfolio that can transfer over to another shirt site. Create a portfolio that you can look back on and say "yeah, I didn't win, but this was totally worth the time I spent." If you're in it for the money, you should be spending enough time to cement that win, and if you're spending that much time on an entry that has only a 10% chance of winning at best, it should be one you'll be proud of. The reason Cho has such a strong fan base is because from most marketable to least chance of winning, he puts himself into it. The reason Edgar fans are so rabid despite his difficulty in securing prints and sales is because Edgar constantly creates what comes to him, not what will necessarily sell. A nondescript and nonthreatening shirt might make you a quick grand, but creating art for you will give you a loyal fanbase, and if you want to really put time into this pastime, that base will be more valuable than a thousand quick cashouts