Upon login, session crashes and returns me to greeter

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http://unix.stackexchange.com – I'm running Elementary Luna Beta 1 and I'm running into some pretty difficult problems. Whenever I try logging in with the greeter (either pantheon-greeter or even lightdm-gtk-greeter), it immediately returns me to the login screen after the screen goes black. I'm not seeing anything significant in ~/.xsession-errors or in /var/log/Xorg.0.log. Where else should I look for potential problems? How can I debug what's going wrong with my session? If I log into a guest session, everything works fine, but I can't log into my own account. (HowTos)