Disclosure: some of these may actually be implemented, but I haven’t seen them.
Smarter Recurring Events
I am often creating events for the same thing, for the same time, in the same calendar and for the same duration, but on different (and varying) days. Instead of offering recurring events, why not allow me to drag a “template” of sorts from a sidebar into a day? This template could save whatever information the user chooses (title, start time, end time, alarms, etc), and it will prompt to fill in the blanks wherever needed (if the end time, for instance, isn’t in the template, it will ask what time the event ends at).
Clothing Store Accounts
I have accounts at various online clothing stores, yet none seem to save my sizes. Why not allow me to define my sizes in my account page, and only show me what’s in-stock in my size (and default to that when adding to my cart). A smarter implementation of this will take into account fit — if the store knows a particular brand of shirt will fit 1 size smaller than indicated, also show me those products.
Automatic Flickr Tagging
Flickr knows when and where I took a photo. But it also knows where and when other people took photos. Why not crowd-source tags? When I upload a photo with geolocation data that places it near the Eiffel Tower, it’s probably going to be a photo of the Eiffel Tower. It would be fantastic to provide a list of frequent tags in that area, and allow me to add those with one click.
Better still, since it knows when photos were taken, this would work for events, too. If a bunch of people upload photos taken at a concert at a venue on a specific date and time, it knows about that event, and what tags people are applying to those photos.