I really really really like thier conversation threading. Because I get so many recurring transactional emails (eg bank statements, auto pay, etc) it makes it very easy for me to view ALL of these in one thread, and not take up a lot of space in my inbox.

Honestly I wish other providers would give me this sort of 'transactional email' conversation tie up because it's convienent. I can see in the last 12 months for example, I have always paid my gas bill on time, at a glance too!