Granny Weatherwax (several times): I ATE'NT DEAD.

Good afternoon, said Death.

I love the caps lock. :D

That's kind of a hard question... You see, including the YA novels, there's 37 books in the series, and they don't all follow the same character (which largely is how Pterry avoids Sequelitis).

But, broadly, the Discworld books are about life. They're about people. I mean, they're funny fantasy books, yes, but they also reflect our world, and the struggles and hardships therein.

There's books about war, and books about overly extremist/fundamentalist religions. There's mystery books (the Watch books and The Truth), and more classic fantasy (the first two books). There's more than a few with highly feminist slants (notably Equal Rites). There's books about Death and what comes after, books about adventure and overcoming obstacles, and there's even a book about opera. There's a book about the newspaper, and a book about the postal service. Every book has anti-prejudice themes. 

They always end happily, though. Or at least end in a better situation than they started with.

And they're hilarious. Like, laugh-out-loud funny.

They're not difficult to read, either; each book is about 300-400 pages, and the plots go wickedly fast.

There aren't really any bad themes, if what you mean by "bad themes" is content of an explicit nature or gory details. If there's a gruesome death, Pterry will make sure you know it's gruesome without going into detail at all. Like, for example, in a passage detailing the murder of an assassin with a still-red-hot sword, all he says is "There was a short scream, and a very long hiss." And that's it. Similarly, there is nothing, ahem, explicit as a general rule, and if it becomes necessary oblique inferences will be made. I'd deem them very safe, unless you have an extremely active imagination.

As for starting points, I'd recommend Guards! Guards! (the first of the Watch books), Hogfather (a good introduction to the wizards and Susan, though in the middle of the Death books), Mort (first of the Death books), Going Postal (first of the Moist books), or Small Gods (stand alone). Don't start with Thud!, Night Watch, The Fifth Elephant, Jingo, Feet of Clay, Maskerade, Lords and Ladies, or Unseen Academicals, because they won't make much sense if you haven't read the earlier books.