Notifications are used to disappear, it would be fine if they will be saved somewhere. Something like this but for xfce, not gnome http://www.makeuseof.com/tag/review-notifications-ubuntu-desktop-linux/
I would love to see something like this as well. As described on the included link would be a good implementation. However, my dream would be something like what Gnome3 has now. You can see an example in this blog post, under "Better Notifications" : http://www.omgubuntu.co.uk/2015/02/4-reason-why-gnome-3-16-might-be-the-best-version-yet-gallery It doesn't necessarily need to look that way, but to integrate Orage and a clock, perhaps together with reminders of upcoming events would be perfect. Maybe various elements in it could be optional, like plugins, so it would be possible to only have a list of notifications or have notifications, calendar, time, etc. That way new content could be developed a bit at a time, for those interested in adding to the functionality. I realize this is a big project, but I'm willing to donate some money for it. Perhaps someone else is too, so I'm setting up a Bountysource-entry. :)
@Yan Pas: I have added a panel plugin to xfce4-notifyd now (currently in git master, hopefully soon to be released) and this is as much a "notification center" as I believe would be appropriate with notifyd. @Daniel Eriksson: I think your requests go way beyond that scope, but at least if someone wants to build something like that there is a reference implementation of the notifications part of it.
Great news, Thanks!