IMHO, Pratchett (once he really got going) is one of Britain's best writers .
He somehow manages to combine ludicrous fantasy with characters who have real depth, themes covering deep and important issues (some of which can make you weep), with page-turning plots - and yet manages to have you laughing hysterically at the same time. Walking along listening to him generally gets me some very odd looks as I burst out laughing (mind you, Neal Stephenson's 'Snow Crash' is getting me the same response).
To quote Shakespeare (who was talking about another clown):
"He uses his folly as a stalking horse
And beneath the presentation of that
He shoots his wit."
Another tip - the Discworld books move between different sets of characters, and each set has a different flavour. Some may appeal to you more than others.
There's the books around the wizard Rincewind - Colour of Magic, Light Fantastic, Sourcery, Eric, The Last Continent, Interesting Times. These, to my mind, are the weakest set (though the last named is still pretty good).
Then there's the set around the Witches, including an amazing character called Granny Weatherwax - Equal Rites, Wyrd Sisters, Witches Abroad, Lords and Ladies, Maskerade, Carpe Jugulum. These are excellent, particularly the last, which has some very grim moments.
Then there's the ones circling around the personified Death (who rides a pale horse - called Binky): Mort, Reaper Man, Soul Music, Hogfather.
And then one ones about the City Watch, which are almost in a jokey Film Noir style, featuring Captain (later Commander) Vimes and Corporal Carrot, who's a six-foot seven dwarf. These include real mystery story plots (among many other things) - Guards Guards, Men at Arms, Feet of Clay, The Fifth Elephant, Night Watch. The latter is almost not humorous at all, but very moving.
And then an assorted set: Pyramids, Moving Pictures, Small Gods, Jingo, The Truth, The Thief of Time.
All of these books share the same basic background, and there's lots of cross-over between the sets of characters, particularly the wizards, who turn up in a lot of the books.
Lately Pratchett has been taking some interesting turns - almost out of the Discworld canon but not quite - with a more serious aim, I think. The Amazing Maurice and his Educated Rodents, The Wee Free Men and Monstrous Regiment.
Not all of these are so far available as audio books, of course, but many of them are.