The dash is the top icon on the launcher. Originally, it was a combination menu and file manager replacement, and no more than adequate at either task. However, in the last few releases, it has also become an instrument for searching online, especially on commercial sites.
This combination has been tried before -- for instance, with KDE's Konqueror -- but it has only succeeded on the Chrome desktop, which emphasizes online services and minimizes local utilities. In Ubuntu, the combination only distracts. When you are searching for an app or a file, who wants recommendations for reading or listening?
Even worse, the online searches raise obvious privacy issues that Ubuntu has only partly answered. They are a feature that very few want, and Ubuntu's insistence on not only keeping them but expanding with each release to the number of sites you can search seems to indicate how desperate Canonical, Ubuntu's commercial arm, is becoming in its search for revenue.
Xfce's most outstanding feature is often said to be its speed. However, now that the feeblest computer sold has several gigabytes of RAM, Xfce's speed is less important than it once was.
Xfce has a powerful file manager in Thunar. However, in other areas, Xfce's native tool selection is sparse: a half dozen utilities, and only the most utilitarian panel applets.
Perhaps this sparseness is necessary to keep Xfce efficient and lean, but it can also be a nuisance when you discover the lack of a basic tool while in the middle of a task. Fortunately, you can borrow from GNOME or KDE as needed, but the lack is a frequent nuisance, all the same.
This list is nothing if not personal. I know KDE users who are indifferent to the Activities that I use daily, and Ubuntu users who hate Unity's launcher as much as I appreciate it. In fact, I believe that one of the most popular Unity extensions is one that moves the launcher from the left side of the screen to the bottom.
Similarly, in many cases, I could have offered alternatives to the features I settled on. I value KDE's FolderView, which quickly loads icon sets almost as much as Activities, and I could have mentioned the default maximized windows in GNOME as a pet peeve rather than the overview
But, then, preferences in desktop environments are nothing if not idiosyncratic. What features do you like or dislike most in your desktops?