We also made the design decision to allow notifications to be hosted by the system tray applet. That has changed somewhat: the notifications agent is now a stand-alone Plasmoid, so you can put it anywhere you wish. By default, it lives with other items in the system tray, which is sort of a panel-in-a-panel (or a panel-on-the-desktop if you use it there). We've been working on making that more elegant for 4.5 as well.
Here is my system tray as of today's build of what will one day be KDE 4.5:
You may notice two things immediately: the icons have changed in both appearance and ordering. We're still experimenting with all of this, and any of it may change at a later point. For instance, I think it's likely that we'll end up changing the technical implementation of the how the new icons are sourced slightly. So, it's a work progress; please keep that in mind as we continue on here. :)
As for ordering, we're experimenting with putting the "most important" entries on the left of the system tray and the least important ones on the right. While this feels "good" visually and makes sense from a reading order POV, we're still unsure whether it's the "best order" due to the possibility of icons moving around in a way that increases the likelihood of changing the order of the "most important" icons on the left. We'll see.
In any case, entries can label themselves with a status, such as "Active" or "NeedsAttention". Both Plasmoids and the new system tray protocol items can do this, and the system tray Plasmoid listens up for these changes and arranges the icons accordingly.
It also groups icons according to type: application icons are kept together, hardware icons are kept together, messaging icons are kept together ... This reduces some of the chaos. With the advent of the menus being client side, something Aurélien Gâteau contributed as part of his work for Canonical, we may end up booting out the application entries from the system tray in the 4.6 release (we likely don't have time to experiment with this for 4.5, sadly) and instead host them in the task bar along with their windows. Towards that end, we have a new DataEngine for status notifiers contributed by Matthieu Gallien (with some code borrowed from other works by Marco and Alain Boyer in the mix), which will make this quite a bit easier.
The non-application icons are also losing their colorful nature and becoming easy to see and understand glyphs. The battery Plasmoid also doesn't save extra space for itself for the on-hover display of battery power. Tooltip hovering is consistent as well. I can now swipe my mouse across the tray and quite easily and quickly see the status of my battery, internet, sound, screen and more in an attractive manner.
The "more icons" arrow has moved to the right as well and now opens a window with all the entries in them, looking all Plasmalike. This prevents rearranging everything on the panel (or simply running out of space on small panels!) when expanding the system tray.
The information icon for notifications and job progress has also been toned down. Not only that, but it has been augmented with a number of nice animations and state transitions to communicate what's happening. For instance, when a notification comes in, it smoothly animates between the unobtrusive "empty" icon and a full coloured one (blue by default):
The transition is very smooth (hello, QtKinetic and Plasma Animations!) and while eye-catching isn't vulgar during its movement either. Another case where it moves is when one or more jobs are in progress, such as downloads. When they first get started, it shows the new (well, to Plasma, you may recognize it from other places :) busy animation along with a count of active jobs:
I caught it at a poor moment and so the snapshot looks worse than it tends to in practice. (Yes, I was too lazy to do a screencast today. :P) What's really cool is that it starts to fill up pie chart style as the jobs progresses:
Clicking on the icon expands to a fuller job information view, with multiple jobs showing in a scrolling view so that they don't eat up the screen anymore. Same for notifications, by the way! Clicking on the icon again will make all the information hide and you can just watch the icon slowly fill up, at which point you get a very stylish notification letting you know what was finished along with a link to click to open it and a button to copy the destination with.
There are also fewer lines, backgrounds and what not in the default theme for the system tray for 4.5. Other themes are still able to do so, of course, thanks to the Plasma theming system, but we're aiming for a less-is-more elegance on the visuals.
Between the better visuals, the nicer interaction, the increased interoperability (e.g. Gtk+ apps that use the DBus Status Notifier system will appear 100% natively rendered in Plasma Desktop and Netbook!), I'm really happy with how it is shaping up. We still have some more icons to do (you can see krandrtray's blue monitor there still, for instance) and there is work left to be done to integrate the status notifiers into the taskbar, but we're getting there step by step, release by release and the results are very satisfying in day-to-day usage. It feels good, it looks elegant and it lets me do what I want and need to. :)