I still haven't read the non-Discworld books. :-\ I'm sure some people here would be either shocked by this, or ask "What are you waiting for?" or say "HERETIC! BURN HIM!"
But I came into Discworld almost sideways, I guess. I started with
Interesting Times, which was interesting (pardon the pun) but I was a bit lost because I had no idea who Rincewind or Cohen the Barbarian were. :-\ After a bit of a hiatus, I started reading other Discworldy books, got hooked, started scouring 2nd-hand bookshops for the earlier books, and the rest is, etc.
Of course I know Terry wrote books before Discworld, but part of me is afraid that they won't be as good. *shrug* At my local library, Strata and the Johnny books etc. are in the "Children's" section, which forms a little barrier all of its own. (The last time I went to the "Children's" section was to find Asterix comics for my niece, and I was frowned at, as if the parents reading to their kids were like: How dare you?) =P
I also tried to find the pre-Discworld books at bookshops, but they don't seem to exist anywhere. It's as if there's a great conspiracy around them to stop people from buying them, which is strange. (I can't find the Long Earth books in bookshops, either -- does anyone know what's happened? Did they simply not sell as well?)