While this is not exactly an answer, I can contribute describing the differences between Keynote, Impress and LaTeX Beamer. Previous answers only seem to have second hand knowledge of what Keynote can. What makes Apple Keynote "so cool" are the smart grids which lets you align stuff with great efficiency and speed. It also handles all kind of media files and is still extremely fast at it. In OpenOffice/LibreOffice you need much more clicks and submenues to align graphics or text. Once you have added images which have a slightly higher resolution it becomes unusable and slow to the point that you just hate to work with it because you are spending time on waiting for it to save the document. Trying to embed video files is even worse. The LaTeX Beamer class templates look terrible scientific and are not usable for presentations outside academia and research just because of thir graphic design (notable exeption: hsrmbeamertheme has a refreshing look). Sure you could make your own styles, but typically presentations are done with very little time, and you just want to throw some things in, not modify styles just to see how it looks when you have three pictures on one slide instead of one. This is where Keynote shines. Efficiency, ease of use and speed. Scala was praised here, but is a digital signage solution, I'm not sure if it even compares to presentation software at all.
I currently use Scribus to make PDF documents, which I present using the pdf-presenter-console package, which is a great little tool. I have no solution for adding videos though, which can be a huge drawback compared to Keynote.