I had a Hotmail account for about a year in 2000 or so. It had a 10 MB inbox and worked quite well. I got a bit of spam, but it was obvious and manageable.
My girlfriend got a GMail invite from a friend and sent me an invite. It was huge. A 1 GB inbox, built-in Google search of your email, and incredible spam filtering.
They marketed it something like "With GMail, you don't have to delete your emails. Just search . . .". Really killer features compared to their competitors.
Later on, they added automatic email thread grouping which made using Outlook for work a chore until it got a similar feature.
