Hotmail and Yahoo mail were the primary contenders at the time. Both featured banner ads at the top. Because CSS and HTML were in their infancy, the height was generally fixed, and on smaller monitors it could take up 1/3 of the screen or more.<p>As I recall, Google had no ads. A small text ad, maybe, but I think that was added later. They also did a much better job of removing spam from my inbox. In addition they offered a lot more storage for free, which was actually a concern at the time. I'd have to go through and purge my email because simple emails with images were forcing me to manage my old emails. This became a problem over time with Hotmail, which I used, because I wanted to save some emails for their historical value or value to me.<p>Gmail jumped right on common features like folders and tags, and continued to expand further than other free email clients with better search functionality, easier multi-account SMTP and POP management through a single account, which I use for my 15 email accounts to this day, and automated filters that allowed me to make sure I saw certain emails or didn't see others unless I wanted to.<p>I think for most people it was just a clean interface, more storage, and better spam. But it also had features for more advanced users that made both tech amatuers and pros prefer it.