Denethor apparently was a great man, but damn unsympathetic in his actual book appearances. The big reason was his favoring of asshole Boromir (rhymes with boorish) versus noble Faramir (rhymes with fair). Not only did he favor Boromir, he sent Faramir out on a pointless, suicidal mission.
And said a lot of mean things to him.
That doesn’t make for a sympathetic (or well-rounded) character. Apparently the stewards nobly and thanklessly defended the border, but that’s all backstory, apart from the actions of Faramir who Denethor tries multiple times to kill.
Denethor was understandably hesitant to turn over the great kingdom that his family had ruled for 25 generations. In any non-fictional universe “stewards” gets changed to “defender”, “ruler”, then “king” in about six generations.
The movie version didn’t bother me much, because I had a lot of more serious problems with the films, but – in a world where aspect (or appearance) is so closely tied to character, Denethor should have looked more like a movie star. In the book he was noticeably more majestic than Theodan, whereas in the film he was Rasputin.