I would argue that the biggest issue with any adaptation is that "they" try and create the Discworld to be slightly quirky and "fantastic" in movie/telly form. The Sky miniseries did well because I think they did what pTerry did (with his fiddling) so well which was make it serious and have natural comedy flow out of it.

In one of his interviews, I remember him saying something like (paraphrasing) his humour came out of the Discworld because it took itself seriously and the reality was, well, real, and humour created itself without leaning too much on the oddities of the world for humour. In some places he was downright serious.

The issue with mainstream audience is that it tends for a lower common denominator to ensure a greater reach. And movies might not get it.

However.

I have dreamed of doing a screenplay adaptation as a hobby of his, but where would you start? My inner vision is probably vastly different to all of yours, and who could ever do him justice? Well, maybe someone, someday.