Sure, TWFM is a great introduction to a new reader.
"Nation" is a good stand-alone, even if it's not Discworld. Perhaps it could be recommended to any reader who turns up their nose at fantasy! Because surely everybody know that fantasy is only for geeks in anoraks!

And so on. (You know, all the hoary old cliches about fantasy readers).
I'm not sure where "Eric" or "The Last Hero" would fit. "Eric" is very lightweight, but I suppose it could please a new reader who thinks in terms of "Let's take a quick look at this Pratchett bloke and see if he's worth it."
"The Last Hero" is similarly short, but much better value, in my opinion. It's not just the very fine illustrations, but the story is stronger and the characters are more compelling. (The fact that I just listened to Stephen Briggs narrate TLH has nothing to do with it, of course).
What do you think?