Shortcut keys, auto-complete, custom-GUI elements for which no comparable standard element exists, drag and drop, image insertion/resizing, rich text formatting, auto-updates for new items, etc.

It's possible to make a web 1.0 HTML email client but GMail is much much better. It's the difference between 1998 MapQuest and Google Maps.

There used to be a basic HTML version of gmail but it seems it just redirects you to normal gmail now.

No one in their right mind who does a lot of emailing would use it compared to the normal one besides for ideological reasons.