I think a lot of his themes focus on domestic violence (Insomnia, Rose Madder, Dolores Claiborne) and alcoholism (Tommyknockers, The Shining) and helplessness (Cujo, Misery, Gerald's Game). To name a few of each.
Lots of his books link to the Dark Tower series. And it's definitely worth persevering with as a series. You don't need to read it to enjoy his other books but when you do, a lot of things in the other books have a kind of extra "a-ha!" level about them. Like being in on the joke or the secret.
Some only have a tiny hint at DT links but Desperation, The Regulators and Insomnia all have a lot in common with them. And Insomnia plays and important part of one DT book later on in the series.
I love it when a character from one book shows up unexpectedly in another one. Which does happen often in the DT series but it's hard to say without spoilers.
The book due out later this year, Doctor Sleep, continues the story of Danny and some of the other characters from The Shining (including dead shower lady!)
And I had a lovely moment last week reading NOS 4R2 by Joe Hill (Stephen King's eldest son) when even he referenced his dad's books more than once as part of the story. I really recommend this one actually, it's his best so far.