Things that I really like about Zen:

• New tab URL input, it's a dialog overlay
• Tabs opening as the first instead of the last in the sidebar
• the tab sidebar
• opening external links is magic, they open in an overlay with the option to expand in a tab. This is probably my favourite, because often I need to open a link from an email, do one thing, then go back to the email. This feature keeps me in the context of what I was doing.