It only took Flickr four months to get around to it, but user-created videos now sit alongside photos as part of the widely-used slideshow tool.
Previously videos, which are still, resided in their own island. If you were putting together a special set for a slideshow, you'd have to open up the videos separately. That problem no longer exists--much to the bemusement of vacation photo and video enthusiasts.
Flickr has also increased the presence of slideshows around the site, including where you can fire one up. This is most noticeable on search results and people you're not friends with on the service. The UI has also been improved to scale smaller photos and videos to fit the screen as it sees fit--something the company is calling "embiggening."
What might be the most useful improvement however is that you can now embed a slideshow outside of Flickr. There had been a handful of third-party tools to let you do this, but nothing official--or easy. You'll now find a "share" option on the top right hand corner of any slideshow, and you can grab that same code from within an embed too.
Below I've put together a small collection of photos and videos in case you want to give it a spin. Be sure to toggle the full-screen option (in the lower right hand corner) to have the photos scale to your display.