(a) The "We're deleting all your content" message was only visible on group home pages, not when viewing messages, meaning a lot of the people who used the web-interface didn't know about the shutdown.
(b) The email sent out to all members about the deletion was sent weeks after the initial notice, often didn't arrive, and was inaccurate (dates had changed by the time people received the email). So many folks that used the email interface also didn't know about the deletion.
(c) The amount of time given was far too short. Yahoo Groups had existed as a brand since January 2001, and was based off Yahoo Clubs / eGroups, both of which had existed for several years before that. Two months notice for a 20+ year old service is laughable.
(c) The site has been unreliable during the last few months, due to the high traffic of people trying to save their stuff. This has significantly complicated efforts to archive it.
(d) Yahoo's solution to the data destruction, their "Get My Data" feature, is poorly thought out and often doesn't work. No photos are included in the download unless uploaded by that account. Many groups had members who uploaded photos / scans / schematics / etc.. and since left the group. Those photos are now lost. We've also found the downloads to be missing content that necessitated the request be made multiple times until you could patch together a complete version from all the incomplete ones.