This is good. You like Gmail. What one better feature would entice you to leave Gmail. Only no ads?
Enticing the average HN reader and enticing the average Gmail user are two different problems.
And it already has great spam filtering and minimally-intrusive ads that people have learned to ignore.
Email only sites like Gmail probably won't last in a world where a high percentage of your contacts are on your social network of choice. I want to be that social network and I want to offer a better way for people to message each other.
"I want to be that social network" - if that's your sole strategy, you're screwed. You need to start smaller than that.
Build a tool which lets users do more for less effort and it will be easy for them to adopt. Worry about penetration from there.