That's an objectively incorrect statement. It had:
1. 1GB storage when every other email provider offered like 50 MB at best
2. Superior search
3. Conversational/threaded view for emails
4. Better spam filtering
5. One of the first "dynamic" web applications to use that newfangled "AJAX" stuff to be more responsive and seem faster
Every other email provider was forced to start offering these features and now they seem commonplace. Reminds me of that old joke where someone goes to see a Shakespeare play and complains it was just one cliche after another.
1. _much_ better spam filtering
2. _much_ better searching
3. a _lot_ of storage (no need to delete old emails)
I'd say that its search-first paradigm to mail handling (and related to that, labels) was rather unusual at the time, and for the most part it still is.
Disclosure: I work at Google and get my regular exposure to GMail from there. I run and use my own mail server for everything that is not work.
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.