At the moment I have a metric ton of apps in it (I use pretty much all of them):
* System Volume * CPU/Wi-Fi module temperatures * Clipboard manager * Screenshot manager * Ingress/Egress traffic values (KB per sec) * Current CPU frequency * Battery charge indicator * Network Manager applet which shows Wi-Fi signal strength * Audacious Audio Player * BlueTooth (Blueman) applet * Weather conditions + temperature * Keyboard layout indicator * Time
Remove the system tray from me and my workflow is finished. I will no longer be able to use/manage my devices effectively.