IrfanView is trying to create new and/or interesting features in its own way, unlike some other graphic viewers, whose whole "creativity" is based on feature cloning, stealing of ideas and whole dialogs from ACDSee and/or IrfanView! (for example: XnView has been stealing/cloning features and whole dialogs from IrfanView, for 8+ years).
Features:
Many supported file formats (click here the list of formats)
Multi language support
Thumbnail/preview option
Paint option - to draw lines, circles, arrows, straighten image etc.
Toolbar skins option
Slideshow (save slideshow as EXE/SCR or burn it to CD)
Show EXIF/IPTC/Comment text in Slideshow/Fullscreen etc.