Incredible rythmic aggression (it was 1970 for perspective's sake), perfect banging fast riff, great wailing, arguably a game-changer for rock speed and brutality at the time (with its spiritual twin Speed King). These are the reasons Immigrant Song is "great" in my opinion, plus it's one of those tunes you forever remember the first time you heard it.
About WildRanger: I don't believe him to be a troll (or maybe my idea of what a troll makes is a bit off). He asks questions and I've always read him like someone interested in some kind of intriguing opinion-data collection inside a sort of elusive great unified theory of rock music. The fact he rarely argues the answers to his questions doesn't annoy me in the slightest. On the opposite, it may very well be a hint of a honest need for opinions vs a provocative need to dispute them.