Way faster and uses less memory. Unified inbox. And the killer feature for me is Command-Tab - being able to keyboard switch from my email to the browser.
I was one of those passionate users on Geocities, to the point where I was given shares in the Yahoo acquisition for my work in the community. GeoCities died on the vine because they didn't continue to innovate on the site building and community they had. They certainly could have been so much more - foreshadowing blogs, social networks, everything myspace became, etc and could have found a way to build the kind of sticky (and huge) audience that monetises better - if they'd continued to develop.