It looks like the people in charge of Gmail Labs have decided to do a little spring (or more accurately, late winter) cleaning. They've removed a full 11 experimental features from Labs, with six of the bunch becoming regular Gmail features and the other five failing to make the cut.
Many of the "graduated" features are ones you're probably familiar with, or can at least guess the functions of. Forgotten Attachment Detector, YouTube Previews, Custom Label Colors, and Vacation Dates all fall under this heading.
The last two approved features are Search Autocomplete and Go To Label, and a post on the Official Gmail Blog covers the only tricky parts, explaining, "If you have keyboard shortcuts turned on, type 'g' then 'l' and instead of getting the old 'Go to label' pop-up, you'll be in the search box with the 'label:' operator filled in for you. Start typing the label you want to go to, and autocomplete will take it from there. All you have to do is hit enter. If you want to send your cursor to the blank search field, the keyboard shortcut '/' will do it."
As for the experimental features that've been dropped, they're called Muzzle, Fixed Width Font, Email Addict, Location in Signature, and Random Signature. And a lack of popularity is the main reason they didn't make it further, in case you were wondering.
Anyway, Google's likely to introduce more Gmail Labs features to replace all of this stuff before long.