Product design is what something does, how it is structured and how it behaves to user interaction, _and_ how it appears, not just one of them. GMail was almost to email what the iPhone was to smartphones.
This is product design.
SUPERCUT Every "And loving it" in Get Smart (1965-1970)
If I remember my thoughts back when Gmail was launched I was clearly awestruck. Gmail showed what was possible to do if one had fast internet connection. All those little interactions enabled through ajax! I think it was the first time, atleast for me, a product was as good as a desktop app w.r.t interactions and usability.
For comparison, with Yahoo mail one had to hit compose button and wait for a few seconds for compose window to open up. With Gmail it was extremely nifty and nearly instantaneous because I think it was done clientside and Ajax.
It popularized instant search, tagging and threading to such a degree you have clearly forgotten how elegant it was!