I added below two suggestion the to list of topics to increase the usability of the gmail.

1. Provide Show/Hide link(or button) for the sidebar.
2. Provide the Preview pane to quickly read mails.

Provide Show/Hide link(or button) for the sidebar.
By toggling this link we can get maximum space for the mails so that we can get more space for reading mails; which further can be useful for my second suggestion.

Provide the Preview pane to quickly read mails.
I’m a big fan of Gmail, and prefer to use it over Outlook, but the feature to preview a received
mail would be most welcome. I get tons of spam and unwanted mails from friends as well (chain letters, etc.) and its boring to have to open them to see if they are relevant.It is a critical attribute to working productively. Gmail just gets increasingly more frustrating by the day as I have to click into a message and back out. Its unnecessary.

My suggestion is to make the inbox window splitted into two either horizontally or vertically (like outlook client). one pane could be used for listing the mail items another one for preview the mail. Single click on the mail subject opens the mail in preview pane. (user has all the options like forward, reply, reply to all, delete).Double click on the mail opens in full window. The proposed functionality is same as the outlook client , i hope this feature would  improve the usability of gmail.

If you have any suggestions post it in google lab suggestion.