One of my favorite features is also one of the more subtle ones. When you have an attachment, it won't merely display the generic attachment icon, but will display an icon that identifies the attachment, such as the Word icon for Word documents. In addition, it shows right at the top of the Firefox tab not just the total number of messages you have, but the number that you haven't read.
Better Gmail 2 is an update of the original Better Gmail extension, written to work with the overhaul that Gmail was given after the original extension was written. So, for example, it kills its skins feature because Gmail itself now has skins capability. It's also fixed a number of problems that the original Better Gmail extension had when working with the new Gmail interface.