I'm obviously in minority here, but I don't stress too much about the design as long as unread messages are in bold text. But this might be due to me working mostly in terminals and text/code editors, where design never was a top priority.

The things I care about most in gmail are:

  1. reliability, speed and lots of space
  2. good spam filtering
  3. web view of various office documents (I used to cringe when someone sent me .doc or .xls, not anymore)
  4. fast search which also includes gtalk conversations (invaluable)

If one or two of these things goes away, then I'll probably switch.