WordPress 3.9 brought a huge improvement to the WordPress theme browsing experience. By default, the new browser shows large preview images of themes with indicators for the ones you already have installed. It also allows for better filtering and exploring of featured and popular items.

These updates to the theme browser make it far more likely that WordPress users will want to look for new themes without leaving the admin. The experience is much better than searching directly on WordPress.org.
See More Themes
Themes and their previews load quickly from WordPress.org, but the size of the thumbnails prevents you from seeing more of them on the screen at once. See More Themes is a new plugin, written by Sean Davis, that allows you to browse more themes. When activated, the plugin modifies WordPress’ default admin CSS for the theme browser, reducing the size of the thumbnails to display more themes.
The default theme browser looks like this:

When you have See More Themes activated, you’ll be able to take in more themes at a glance:

Sometimes it takes hours of browsing and previewing themes before you can settle on one that works for your site. See More Themes helps you browse through them faster while still presenting a decent sized thumbnail. It also maintains the theme browser’s responsiveness. If you like being able to see more themes at once, download the plugin from WordPress.org the next time you’re on the hunt for new themes in the admin.
I always buy mt themes but that is a great feature and will make it much easier to pick a theme.