The main theme of his videos has mostly been science vs creationism.
Thunderfoot is also a strong advocate of free speech and has been a valuable contributor in the Youtube Vs. Viacom debate as well as other copyright debates and free speech issues.
I've been a subscriber to thunderf00t's channels for over 2 years and find his videos very interesting and educational. I think the first thunderfoot video I saw was one of the "Why creationists are laughed at" series, where he responds to the various attacks on science by creationists. I also thing these are the videos that made him so popular on youtube.