- Calendar/Events with remindersI'll agree with this one on top. Reminders are useful in most of my types of groups.
Assuming the existence of general Files storage (not on your list), if a reminder can have attached files that might check off the "scheduled files" feature in a more natural and more flexible way than Yahoo Groups' mechanism.
- Photo sectionI might put photos ahead of polls. I had long hoped that Yahoo Groups would integrate their Photos section with Flickr in some slick way, allowing me to present photos from my Flickr stream in a group album without having to upload it separately. Perhaps even creating an automatic linkage of some kind between a set on Flickr and an Album in a group.
- Links and/or wiki (I'm leaning towards a wiki over a links section,I'd definitely choose a wiki over a links collection, if I had to choose. I long ago suggested a wiki as a feature Yahoo Groups should have. And then started using pbwiki because they didn't.
Even though one can manage a links list within a wiki, that is a bit of an artifice and won't satisfy all users (or use cases). But maybe if the wiki has a sortable table as a formatting element it would.