There are characters like JR's son where the audience has a certain expectation how James should be portrayed. But as always it comes down to the writing.
Of course, it was a huge miscast, but I mean how can you come up with "Beaumont" as the last name for good old JR's son. The motorcycle and all the rest...it was just wrong on so many levels.
TNT Dallas (imho) managed to do a better job with JohnRoss.
It's hard to tell if the last +2 years suffered more from miscasting or poor writing, probably both.
I just rewatched S5 (DVD6). It's just so cleverly written. It's not just the central plot about the fight for Ewing Oil with all its repercussions, it's the care for detail (realistic oil prices, Puerto Rico papers, JR buying the same Mercedes again after SE crashed, Raoul&Teresa going to the movies etc.) but foremost you did not know where the storylines were going. All these insinuated plots, like SueEllen and Clayton with Miss Ellie's insecurities, the role of Katherine or Holly Harwood, the introduction of Mark Graison... The secondary characters were all used in a meaningful way, like Harve Smithfield, Punk, Aunt Lily, Fenton Washburn or Dr. Danvers as well.
And the additions worked as three dimensional no matter how small their screen time was, like the McLeish brothers or Walt Driscoll for example. I especially love the dialogue that season. It feels like a completely different series years later.
In regards to your thread, I blame the writing foremost, but still my choice is James Beaumont.