Hidden data in this beta reveals the upcoming existence of Steam Wallet Vouchers
. It also has some hints on "Big Picture Mode".
(To see for yourself, look in the resource files, such as "public/steamui_english.txt" and "public/url_list.txt")
There will be a link to "Redeem a Steam Wallet Voucher..." in the "Games" menu between "Activate a product" and "Manage gifts". It will link to "%store%/account/redeemwalletcode/
), but that currently redirects to the homepage.
There will be a "Big Pucture Mode" button near the Detail/List/Grid icons. The icon for the button will be arrows pointing to the 4 corners, like a standard "enlarge" icon. The "Fullscreen" command the button calls doesn't do anything yet, and the button isn't visible.