I'm fairly well-versed regarding ROMs, and actually ran CM and MIUI on it for quite a while with decent results, but that's not really the point. Nexus is supposed to mean "first in line for OTA updates", not "wait for us to put it on AOSP and then build and flash it yourself, or wait for a modder to make a decent ROM".
That is absolutely not the brand I thought I was buying.
Edit: Sorry, that was a ruder tone than I intended. I've got a bit of a love/hate relationship with my Nexus S due to the OTA frustrations, but I do love that Android has the option for taking matters into my own hands.
The tone's fine, it's not a great response to your frustrations. Carriers and phone vendors see additional support costs for no benefit. At the very least, Nexus does mean that you can get everything you need to build an image from Google directly.