There isn't any room for improvement in Gmail? Really? It's still using the same basic design of every email client since forever (labels/mailboxes at the left, inbox shows a time-sorted list of messages, click through a message to read).
Meanwhile Sparrow showed what was possible if you sit down and at least tried to rethink the email interface -- with things like gestures, or by shifting towards streams rather than lists.
... which still doesn't excuse them not running with the idea and continuing to improve. Sparrow did; maybe I should have said "shifting further".
(And yes, Gmail popularized conversation view, but it was talked about at IBM Research and Microsoft Research years earlier. Of course, those guys are even worse at delivering innovative interfaces.)
Gmail is pretty much perfect and the Labs additions can basically give you a "Sparrow-like" window and viewing pane already.
APPLE needs these improvements, not google.