Username or EmailPassword
You beat me to it
Of course, one can not expect to find all the supported "objects" that PowerPoint supports (like embedded video)
Actually, although you can't do true embedded video, you can make a link that starts an external video app (or any other app for that matter). Not with prosper (which uses the old tex->dvi->ps->pdf route) but with any of the plethora of latex presentation packages that are based on pdflatex (I would recommend beamer).
As to the downsides of LaTeX:
1. It is insanely difficult to get text paragraphs to flow around arbitrarily shaped inserts.
2. As some have already pointed out, LaTeX has its own ideas about what constitutes good typesetting, which may not be the same as your own. With enough experience, it is always possible to get it to do what you want, but many intermediate users get frustrated with this. An excellent resource for solving such problems is
3. Programming in LaTeX/TeX is almost completely unlike programming in other languages. Another source of frustration for some.
4. For ultra-high-end artistic typesetting (think of Bringhurst's 'Elements of Typographic Style'), LaTeX is probably not the best tool, although with some of the experimental micro-typography additions to pdftex it is 99% of the way there.
Apart from these, I'd say it was pretty perfect