I'm running Windows 7 64 bit with 4 GB of ram, ATI Radeon HD 4200 video card with 1 GB of video memory, and the game's process quits after the update window (I know because I watched it in the Task Manager). It also happens in the web browser version, and uninstalling and reinstalling does not work, neither does restarting the computer.

Checking the projectx.log file, I get this:

2015/07/24 05:39:39:155 INFO K$a.b: Logging to 'D:\SteamLibrary\steamapps\common\Spiral Knights\.\projectx.log'.
2015/07/24 05:39:44:196 INFO m.a: Updating Getdown with D:\SteamLibrary\steamapps\common\Spiral Knights\.\code\getdown-pro-new.jar...
2015/07/24 05:39:45:657 INFO K$a.b: Steam initialized.
2015/07/24 05:39:47:822 INFO K$a.b: Couldn't create display; switching fullscreen mode. [fullscreen=true]
2015/07/24 05:39:47:834 WARNING K$a.b: Couldn't find valid pixel format.
Exception in thread "AWT-EventQueue-0" java.lang.RuntimeException: Controllers has not been created.
at com.threerings.opengl.gui.e.f.isValid(SourceFile:638)
at com.threerings.projectx.client.ProjectXPrefs.c(SourceFile:905)
at com.threerings.projectx.client.az.[init](SourceFile:82)
at com.threerings.projectx.client.ProjectXApp.init(SourceFile:1278)
at com.threerings.opengl.o.run(SourceFile:172)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:641)
at java.awt.EventQueue.access$000(EventQueue.java:84)
at java.awt.EventQueue$1.run(EventQueue.java:602)
at java.awt.EventQueue$1.run(EventQueue.java:600)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:611)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

(I had to change the less than and greater than signs around the word "init" to [ and ] to have it show in the post.)