It wasn't enough that their best phone (the Motorola Triumph) had a terrible touchscreen and problems with battery life.
It wasn't enough that, despite it being an unlimited-access pay-as-you-go service, they didn't allow forwarding to third-party voicemail providers, and didn't provide any decent application-based voicemail of their own.
It wasn't enough that the data service was slow and the voice service horrible.
It wasn't enough that they never updated the phone to anything even remotely recent Android-wise, and never released the kernel source allowing the modding community to take it into their own hands.
It wasn't enough that they didn't anticipate that my expiring credit card would cause a problem and prompted me to update it instead of freezing my service when I was out in the middle of nowhere and needed to get in touch with people, and didn't give me any way of finding out what had happened until I got home.
What really pushed me over the edge is that they remotely disabled all of my phone administration applications (Astro File Manager, Gingerbreak, Superuser) and limited my access to my own filesystem.
(Of course, all the stuff above didn't help either.)
Anyway, now I have a T-Mobile G2x, which is quite nice (it's the first Android phone I've had which truly feels responsive), and of course as soon as I get back to San Francisco I will probably be installing Cyanogen on it (although honestly, the OEM ROM is actually pretty nice; it's mostly stock aside from the standard T-Mobile apps, and none of the bloatware/crapware that people have been complaining about on the Galaxy S II).