I think it was the extremely high storage quotas (for the time) and the fact that the UI was ajax’y and didn’t reload the whole page/frame when you clicked.
Spam filtering on gmail has always been OK, but it was fine (for me) on other products for many years before gmail came out. I doubt it was much of a differentiator, but who knows? Maybe the #1 provider before gmail really sucked at it or something.
They were Yahoo and HotMail, and they both had significantly worse spam filtering than gmail. They also had relatively poor security, reflected in bugs that made it common for even relatively careful users to have their accounts hijacked (at least, that’s what I heard from users at the time; I didn’t really use either one much myself.)
Spam filtering on gmail has always been OK, but it was fine (for me) on other products for many years before gmail came out. I doubt it was much of a differentiator, but who knows? Maybe the #1 provider before gmail really sucked at it or something.