I haven't been able to draw anything useful on LibreOffice Draw. Where are the different shapes to make diagrams? All I can find are basic shapes that I would find on Microsoft Word (squares, circles, stars, arrows, etc). Will I be able to make more elaborate diagrams like a data flow diagram or a basic UML diagram?
LibreOffice is a general-purpose drawing application and rather unsuited for diagramming and scientific drawing. It only -- as you have already seen -- provides basic shapes.
For diagramming, GNU Dia is an excellent tool, but there are many other great programmes around.
EDIT: I also want to mention yEd: it scales very well to big graphs and can be run via Java WebStart. It has some really cool features.
If you're after a set of consistently styled isometric network equipment shapes (similar to the Visio ones) try these: http://www.vrt.com.au/downloads/vrt-network-equipment (Creative Commons licensed, in the Fedora repos as and RPM as well).
Now also added to extensions.libreoffice.org and extensions.openoffice.org
There is https://github.com/sschmidhuber/LibreSymbols. But it provides mostly symbols for electric/logic diagrams. In future it will also support eEPC diagrams. May be that's interesting for you or somebody else.
UML is not on the roadmap.
Well they are here ;) So just add em ;)