I bought it personally I love it. As to the blind letters, first you need to click a post office then the more button. One of the things you will be shown is a letter, you then need to decipher the address for example there is one address that merely says ook and I'm sure we can all guess who the intended recipient is or at least the sender to whom it should be returned
However if I had the complete ankh morpork I'd probably think twice, the quests are more just a side issue as far as I'm concerned, it's the extra pieces of info, origin of street names various shop adverts etc that I like.
Improvers would be:
Real time lighting i.e. day & night
Ability to re-view achievements and blind letter recipients after you have done them i.e. random house, vimes residence etc
Ability to enter some of the major buildings (I know that would be a huge ask likewise the following)
A google maps street view type thing (but obviously drawn as opposed to photos) for some of the major streets.
Finally an ability to drop pins on some of your favourite places and maybe even an option to Internet link and add fan art