Even better spam filters? Absolutely no ads?
And, are you already happy with what you can do with IMAP across all of your devices?
Unless you have ideas that are as big as that you probably shouldn't bother stepping into the ring. The product quality bar is very high and so is the lock-in for email (Yahoo/Hotmail are still bigger than Gmail).
Labels/folders, no ads would be awesome, IMAP (and Push) is exceptional.
I'd like to see everyone implementing Gmail's "conversation" layout but not taking it to other level—making email look like chat, that is.
Plus, I'd like the feature to export all of my emails from one account whether it's Gmail or Yahoo, etc.
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.
It could be a way to ensure that emails from contacts with higher votes are prioritized. Is that it?