As much as the capacity or the search, I feel the UI of the original gmail was extremely well done and innovative -- I remember immediately preferring it to the dominant desktop clients of the time, Outlook and Thunderbird.

But sadly, over the years the design has been tweaked and evolved in ways that have lessened the originals snappiness and focus (a problem that appears to endemic to all of Google right now).

So now I'm back to mucking about with native clients, which, happily, do seem to be gaining momentum recently (and Gmail works fine as an IMAP backend, which is very kind of Google). (Though to add insult to injury, Google purchased and then abandoned one of most-promising native email client, Sparrow, which was not very kind!)