1) Remember window position when not in full screen mode (i.e. run in window with a specified size)
2) Hide Task Window to Task Tray when running a slide show
3) Maybe a Zoom Pan transition like Windows Media Center (nice to have)
If the top two are implemented the slide show will certainly replace the Slideshow gadget in Vista and Win 7 and would give XP a desktop slideshow gadget

BTW, I would like to congratulate the people behind XnView. It is an awesome tool

I would certailnly pay if they do the slideshow changes above as it is just about the only thing missing in it.