- There’s a spelling mistake in the basic terms section: "automagically"
Actually, that's not a mistake.
http://en.wiktionary.org/wiki/automagicalhttp://www.urbandictionary.com/define.php?term=automagicallyPerhaps you could also mention that when saving your file you have to manually add the file extension yourself?
OK, added. (Still, I think it would be better if users would not have to add these extensions manually. This has been the most requested feature for Avidemux in the last couple of years.)
An interface image would be a touch too, perhaps with red numbers on it that people can reference too from your text?
Maybe, but we already have the User interface/Main window article, and it may not be the best idea to duplicate too many things...
Would the basic terms be better at the beginning?
Perhaps. I was thinking about it, too, and I agree it should be the first thing the users should learn before using Avidemux. On the other hand, I'm afraid of losing the readers' interest when the Quickstart guide for Avidemux begins with some boring general terminology instead of saying something about Avidemux...
Should the “container format†be the 1st term explained before the rest?
OK, done.
- Finally, maybe a chart showing which container formats can hold which video & audio types (and ones they can’t), & maybe ones typically found too. Also, which container formats can be readily swapped with another/others?
The section links to the Output formats article with its table of supported audio/video formats in different containers. Again, I don't think it would be a good idea to duplicate this in the Quickstart introduction, which should be really short and simple, otherwise nobody would bother reading it.

in fact, I'm afraid it's already too long... In my opinion, if anything in this Quickstart introduction needs a more detailed explanation, it should just link to a separate article, and if the separate article is currently not good enough (like, not understandable to normal people), that separate article should be improved instead.
Thanks for the input.