Caution : wet paint
When reading Discworld series, There is basically two important stepstones :
- Equal Rites/Mort.
- Guards,Guards/Moving pictures.
For each of this stepstones, there is a before and an after.
This is very flagrant in the Light Fantastic : Independent novels linked by Rincewind with full fantasy elements that will never be heard again.
You might even look at the 2 first book as an 'invented story' that either Rincewind or TwoFlowers will later tell.
As a matter of fact, when asking a new reader to start the series, fans frequently advise to start with Mort.
Guards is the first Vimes book, Pictures is the first New Unseen Academy book. Guards is the time Vetinari is really given a character (Pratchett will later admit the previous Patrician is also Vetinari, yet admitting the retcon.. I will try to find the citation).
So basically :
Don't try to compare anything from the 3 first book to the rest.
Be tolerant about what you read in the 7 first book to the rest.
The universe was not ready, the paint was wet.