Actually, Gmail users have been able to view PowerPoint files as slideshows for a while. But now you can view presentations using a fuller featured web interface. The new viewer makes it easier to skip pages, change the image size, and change the page layout of presentations.
The TIFF viewer works much the same way, making it easier to see multi-page TIFF images sent to your Gmail address.
We've had a "View as slideshow" option for PowerPoint files for a while; now we've integrated this conversion technology into the same viewer that we use for PDFs and TIFFs.
Google Docs, will show you every page and give you the option to "print" the TIFF by opening it as a ready-to-print PDF.