Trotsky was basically devoted firmly to "the spirit of critical Marxism" (The Making of Modern Russia, by Kochan and that other guy I always forget to name). So where Stalin and even Lenin were willing to adapt their ideology to remain popular and therefore in power (eg NEP) Trotsky was not willing or able to change from his beliefs-- thus his constant opposition to Stalin, and his consequential humiliating exile.
As everyone else has said, permanent revolution is incredibly important.
I've also been told he was a Populist in his early days, but I don't know what that is. Any help???
The problem with finding info on Trotsky is that he has too many modern supporters. I know that there exist full books on Trotsky, but a) I don't wish to read an entire book on the man and b) I can't access any. So I'm stuck trying to pick information out of fleeting mentions of his name in text/books on Russia or trying to decipher extremely biased Trotskyist tributes off the internet.