I think its natural that you would want to see the entire email, not just the header. The header thing was relevant in the days when most people paid by the MB, but with unlimited data plans, I expect email on the phone to work like email on my PC.
Some people may prefer just the header, and WP7 will not allow that, limited flexibility being one of its drawbacks.
Its not just the gmail id and password that you put in, you also had to enable the setting to retrieve the full email (as you mentioned earlier). Also, maybe this changed in Symbian 3, but in v5, the mailbox would disconnect, and when I got the new mail notification and clicked on the icon, it would first spend 10 secs or so connecting to the mailbox before retrieving the email. Is that not the case in Symbian 3?