Yes! Email conversation threads! I'd forgotten that one. Also a +1 for UX.
You might quibble about this but recognizing that fast search is better than foldering is also UX. It seems obvious to us now but maybe it wasn't that obvious in 2006 (or maybe it was, I didn't know anything about UX or design back then). Yes making the search go fast is a technical feat and without good tech you can't have a good user experience even if the design is good (see WebOS).
You might quibble about this but recognizing that fast search is better than foldering is also UX. It seems obvious to us now but maybe it wasn't that obvious in 2006 (or maybe it was, I didn't know anything about UX or design back then). Yes making the search go fast is a technical feat and without good tech you can't have a good user experience even if the design is good (see WebOS).