Actually, Mail.app's lack of conversation view was one of its definitive dealbreakers for me.

In particular, it doesn't include you or your sent mail in the conversation (http://i.imgur.com/Atkcu.png). It only lets you expand the email quotation to let you dig through nested context (http://i.imgur.com/1Zj2e.png).

Compare this to gmail (http://i.imgur.com/mLel3.png) that actually handles it as a conversation. It's one of the reasons I use Mailplane (thin wrapper around gmail) instead of Mail.app.