There's no one headline feature in the v1.5 update to 2flicka, but it's a comprehensive upgrade nevertheless and well worth going for. As a fully-featured Flickr client, 2flicka takes some beating and you'll have noticed the regularity of update news stories here on AAWP - this is one application that's being very actively developed. Which is just as well, since Yahoo!, which owns Flickr, clearly can't be bothered...
Ability to upload single or multiple pics straight to a photoset
Set cycling lockscreen background (automatically from Friends activity, my stream, favs) or static image that user chooses in photo viewer
Add/change location where picture was taken, to map manually
Ability to set what count should be displayed on the main tile and lockscreen (friends activity or notifications)
Pin explore, search, contacts and groups to start screen
New tiles with accent coloured strip on bottom
Popup toasts for new notifications
Support to enable/disable background agent
Support for High Res picture in viewer
Improved UI to be more in line with WP8 guidelines
Increased the notifications range from 30 to 90 days
Minor bug fixes
Great stuff. I took 2flicka 1.5 for a quick spin:
The UI continues to be polished, even though mating up Windows Phone and Flickr's detailed architectures sometimes seems like a match too far; (right) multi-selecting photos for direct uploading to a specific photo set...
The multi-upload in progress; (right) the photos safely in place in the photo set, ready for sharing as needed...
For the higher resolution devices (e.g. the Lumia 1520), there's the option to use higher resolution versions of photos on-screen; (right) a growing choice of lockscreen display options - 'Friends activity' is a good default though - always something new and relevant!