In my case, I could ssh over to the system and log in fine -- the 'unix part' of my account was ok. Not knowing how the MacOS X 'experience' (desktop, apps, aqua, quartz, etc) is layered on top of that though, I wasn't sure how those layers really work, and where the problem lay in them.

In my case, apps were crashing when I started them up; in that case, it could have been a corrupted higher-level Cocoa library (luckily it wasn't anything this weird), which would still have allowed ssh logins, console logins, or single-user logins without any problem. However, I couldn't create an uncustomized account after I logged in, because along with all the other apps, 'System Preferences' was crashing on startup!

Your suggestions split the problem across the UNIX/'MacOSX experience' line; logging in as the uncustomized user further splits it across the 'MacOSX experience'/user customization line, further dividing the problem. Unless you're a very lucky troubleshooter, anything simple you can do to split the problem further is always good (from my unfortunately extensive troubleshooting experience).