"Go to label" adds a keyboard shortcut for selecting a label. Gmail already has shortcuts for navigating to the inbox (g+i), drafts (g+d), starred messages (g+s), but you can now type g followed by l to get a box that lets you enter the name of a label. In most cases, you only need to type the first letters of the label and then press Enter.
If you like the colors that can be associated with labels, but you find the number of options too limited, there's a new option to add custom colors by selecting colors for background and text.
Probably the less useful experimental feature, "Navbar drag and drop" lets you reorder the boxes displayed by Google in the left sidebar: Gmail Chat, labels, Quick Links, Invite a friend.
The three new features can be enabled in Gmail Labs, a section available in the new version of Gmail and only for English interfaces.
Related:
Experimental Gmail features
Hidden labels in Gmail
{ via Gmail Blog }