I hated Google Groups, the interface seemed so overengineered compared to the simplicity of Usenet. I believe you couldn't even view the (plaintext) posts with Javascript disabled.
I have a suspicion that Stack Overflow's success can be attributed at least somewhat to the experience that was browsing comp.lang.* using Google Groups.
I haven't looked in a while, but for a long time, the single most-voted-for bug in Google's public bug tracker was "give me some API access to the message content in Google Groups."
Ironically, there is already a very simple API mechanism that could have been used to provide exactly all the information people wanted... NNTP. All Google would have to do is provide an NNTP server endpoint to Google Groups (even beyond its Usenet mirror). It's not even that hard to write an NNTP server: it's probably the easiest server to implement of POP3, SMTP, IMAP, and NNTP.
As soon as spam started hitting USENET there really was no way to shut it down. The entire system was held together by "netiqette" and when that died off during the Eternal September, it was only a matter of time before it collapsed.