Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
My main grievance against GNOME 3 and Unity is the elimination of all except a few applets. Both grudgingly allow some basic applets, such as a clock, but each lacks the ecosystem of applets that made GNOME so configurable.
You only have to look at the rich assortment of applets in KDE -- which calls them widgets -- to see what GNOME and Unity are missing. If anything, KDE's widgets are even richer than GNOME 2's applets, with dozens of small pieces of functionality that range from the serious to the trivial and the basic to the expert and obscure, including everything from hardware indicators and system configuration tools to desktop enhancements and toys and educational software.
The exact choice of widgets varies with the release and the distribution, but over 75 are included with most implementations of KDE. You can place them on either the panel or the desktop, depending on whether you click the desktop or panel toolkit (the so-called cashews) on the upper right of the desktop. Select Unlock Widgets then add widgets, and a searchable, horizontally-scrolling window opens.