The thing I missed most switching from RISC OS 6 to RISC OS 5 was the loss of PNG support.
PNG is a very common image file format. It is very useful for sharing bit-mapped images with other platforms. There are 3rd party tools to access PNG in RISC OS, but no support in the OS.
This has now been fixed with one of the latest bounties. This adds a new module (CompressPNG) to the OS (which developers can use to improve their applications), along with PNG support in ChangeFSI and Paint.
This will be available in the next stable RISC OS release, but you can try it now by in the nightly builds from ROOL.