Tenor is an integrated desktop search effort; in the same vein as the Windows-only offerings from Yahoo and Google, Mac OS X's Spotlight, and GNOME's Beagle. Where the Tenor project differs is that its indexing and search capabilities are to be built into the core KDE desktop itself, and not exist as a standalone application.
The Contextual Linkage Engine will run as a service, searching and index the metadata, text content, and linkage relationships between files and other items of interest. The CLE's findings will be stored in a Postgres database accessible via the KDE infrastructure -- both to the user with traditional search tools and to other applications with a new API.
Plasma is an overhaul of a number of related but formerly disjointed desktop technologies: the "kicker" taskbar, panel applets, SuperKaramba desklets, wallpapers, and application launchers. The goal is to unify as many of these disparate functions as possible into a single, simpler entity.
Users stand to benefit from having a unified, consistent interface to these applications. They have always appeared united from the outside, though they were not under the hood. Furthermore, Plasma project developers anticipate that reworking the system will inspire applet makers to build novel new creations that span multiple components -- the desktop, panels, and free floating windows. A new applet-building studio is in development, and should support multiple authoring languages.
Oxygen is a new icon and widget theme, slated to be the default in KDE 4.0. Oxygen builds off of the look and feel of previous default KDE themes, but will be scalable vector graphics, designed to look good at large scale and adhere to important usability standards. Much as the Tango project is doing for GNOME, the Oxygen icon artists are working on specific interface and style guidelines to reduce visual clutter and maintain a unified appearance.
The Oxygen icon set is in preview right now, and like its GNOME counterpart has published public documents illustrating the value of simplicity. Both projects cite the need to design to a predetermined color palette, set rules governing the use of 3D appearance and lighting, and agree on standards for MIME-type documents.
The Oxygen team is working on subtle, "subliminal" differences in color and line weight to distinguish between file, device, application, and tool icons. It also advocates a reduction in the number of icons in application and contexts menus for clarity's sake. Once the usability work is polished, the group plans to work on adding functionality to the icons, including animation and new modes of interaction.
Coolness is Hans Oischinger's crusade to integrate modern video hardware effects into KDE's desktop system. This includes implementing things like translucency and animation, then using them to add meaningful enhancements to the user interface.
For example, Oischinger has posted videos illustrating two different uses of window transparency at the Appeal wiki. One uses transparency to dim inactive windows when cycling through the active applications on the taskbar. Another uses transparency to fade in and fade out windows as they are hidden and shown, moving them on screen or off screen with an effect Oischinger has termed "fleeing windows."
Animation, on the other hand, may be a more suitable replacement for blinking the taskbar or a window in order to summon the user's attention. Clearly, the desktop computing experience can work fine without visual enhancements such as these, but it is important to observe that they are not merely decorative effects; each serves a specific purpose to increase usability.
In all, the Appeal project looks ready to both augment and beautify KDE usage. And although the 4.0 release timeline has not been set, much of the work these projects produce will likely find its way back to the 3.5 series on desktops.