So with the caveat that you need to understand those first two books aren't representative of what the series becomes, you could definitely start at the beginning. Just know that across so many books, there's bound to be some unevenness. Some books are just plain fun, others have deeper messages, some are, frankly, just not as good. Early highlights like Guards Guards, Pyramids, Small Gods, Wyrd Sisters are the ones where you'll discover the real breadth of what he does. Later books can be less consistently original, but when you get to books like The Truth, Night Watch and Monstrous Regiment, there are some real gems.
But don't let yourself not read any of them. That would be a real shame.
Other Discworld stories that surprised me include Lords and Ladies and Thud!, in which Pratchett manages to write genuine horror stories.
For example, the books about the Ankh-Morpork City Watch start with "Guards! Guards!", and the subsequent books in this sequence tell the continuing story of how the Watch evolves into a (slightly) modern police force. If you start in the middle of this sequence, the payoff won't be as good.
Similarly, there's the sequence about the witches ("Equal Rites" etc.), about Rincewind ("The Colour of Magic" etc.), and his last books about the "Ankh-Morpork industrial revolution" (starting with "Going Postal"). Then there are some one-offs, like "Eric", "Small Gods" and "Pyramids".
I tend to recommend "Guards! Guards!" as a starting point, because I find the City Watch stories the most interesting and enjoyable. Together with the later Ankh-Morpork books they read like an insightful origin story of a modern society, and encompass many sociological topics. Several of the Watch books are also highly original detective mysteries.
YES: start with The Colour of Magic at the beginning and work your way through
NO / Not sure: Don't start at the beginning, as the first few books are weird and will put you off.
Question 2: What sounds more fun to you, reading about Badass Witches, Badass Policemen or Badass Businessmen?
Your respective starting points are Equal Rites, Guards! Guards! and The Truth.
I liked the character of Granny Weatherwax but the Tiffany Aching series really brought out her essential nature in a way the other books didn't.
Damnit now I feel sad again.
The Wee Free Men and Dodger are also nice to read. They focus a bit less on humour, being geared a little more towards children. Don't expect to laugh all the time. But the settings of these stories are magical.
My favorites are all Moist_von_Lipwig books. I recommend starting with Going Postal. Its comes much later in chronological order, but is the first Moist_von_Lipwig book and is enjoyable standalone.
Sorry, Sir Terry :(
Extending this comment, Good Omens is an excellent book, but not anything like the Discworld series.
Small Gods is a good discworld book to start on (the best in my opinion, of course everyone will have their own).
Unless I'm missing something important, you're thinking of Good Omens, rather than Small Gods.
I looked and generally agree with that suggested reading order and I started with the Watch series. I'd pick one of those series, give the first book a shot, but if you hate it maybe give one of the other continuities a shot before you throw in the towel. I hated the Rincewind novels and it took me a while to come around to the Witches (now I love them) but I thought the Watch books (starting with Guards! Guards!) to be absolutely fantastic.
He's written a lot of books of varying quality over a lot of time so I always warn people to be prepared to give him a second chance.
I met him once when I was a teenager. He was really kind as well as insightful and funny.
Be aware that if you start at the beginning with The Colour of Magic the very early books are a quite different style to once he'd got into his stride, so unless you're absolutely determined to read them all I wouldn't start there.
Edit: Btw, Guards! Guards! is early enough in the series that it's still recognisably 'early' Pratchett, so if you're interested in seeing his style develop it's a great place to start for that reason too.
As for Pyramids, for me as someone who's first programming language was an 8-bit BASIC variant, and was amidst calculus and trig etc. in his later high school career the camel's thought processes were totally mindblowing.
For all I know, Terry Pratchett accidentally rigged reality through writing this book in order to turn me into a software developer...
--
"The silence that followed was by way of being a standing ovation.
The landscape began to distort again. This was clearly not a place to linger. You Bastard looked down at his front legs.
Let legs equal four.
He lumbered into a run. Camels apparently have more knees than any other creature and You Bastard ran like a steam engine, with lots of extraneous movement at right angles to the direction of motion accompanied by a thunderous barrage of digestive noises.
'Bloody stupid animal,' muttered Ptraci, as they jolted away from the palace, 'but it looks like it finally got the idea.' "
[1]: http://www.randomhouse.co.uk/editions/the-long-utopia-the-lo...
But then he got better. Much better. Much, much better.
Then buy the lot, you will not regret it. Some are better than others, naturally, but even his worst is like Tom Holt on top form.
I'd read them in order, personally. Though that might just be because that's been my experience of them.
As others have said, the style changes after The Light Fantastic, but the first two books do a lot of scene setting and character building that feed into the later works.
edit - Troll Bridge was published in the anthology 'After the King: Stories In Honor of J.R.R. Tolkien' - http://tolkiengateway.net/wiki/After_the_King - buy that too. Is all good.
http://markdownshare.com/view/0d9bab9a-065a-4838-adf5-17d507...
https://www.kickstarter.com/projects/snowgumfilms/terry-prat...
Can anyone who's read through them comment? According to the Discworld wikipedia article, publication order may not be the best way to go through the series. This[1] page recommends reading a plot/chronological order. Personally I prefer reading books in publication order, since I get to follow the mind of the author. Given that, there are a lot of books here so I'm open to suggestions.
[1] http://www.co.uk.lspace.org/books/reading-order-guides/index...
Edit: Thanks for the responses, I had already reserved The Colour of Magic (you have no idea how nice it is to type that as a Canadian ex-pat) at my local library, I'm going to hunt down Guards! Guards! now. :)
Edit 2: And... there's a waitlist. Thanks, everybody!