What are you looking for in a lightweight email application?

Hello. So, like many others, I'm developing my own email application for OS X. But this is different. This isn't going to be your typical run-of-the-mill app that does every possible action you'd ever want to do on your email, and feed your dog as well. No, my goal for this app is to keep it simple. I want to cover the necessities, but with elegance.

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I already have a working prototype I'm quite pleased with, and to be honest with some more improvements I could release this as-is. However, there's something missing. Something isn't revolutionary about it. I don't want this to be a mediocre app – I want this to be game changing.

I've got a couple ideas that I'm throwing around in my head, but I want to ask you, dear readers, what your thoughts are.

What would you be looking for in your dream email application?