- It's effectively a simplification of the 'multi-document-interface' dumped by Microsoft back in 2000 (when Microsoft Office started using multiple windows, one for each document). It's hardly innovative.
- It erodes session cookie security. 'Session cookies' (used to handle secure logins, amongst other things) aren't accessible between browser windows for security reasons, but are between tabs. This means that cross-site-scripting attacks are easier.
- If a browser window dies, all it's tabs die with it.
This blog post was inspired by the fact Firefox just crashed on me, killing 20 tabs with it.
All that said, I still like tabbed browsing .