Good question. For me, being a Western history buff, it was how authentic to the period the film looked and felt; from the guns, holsters, clothes, saddles and Victorian dialog, I felt Hollywood finally got it "right." For many of my cowboy and buckaroo friends (I live in the rural West) it was much the same, including the outstanding horsemanship demonstrated in the film.
For my law enforcement friends it was the peace officer "law and order" angle that got them. For most Americans I think the "Justice is coming" theme resonated with them. After all don't we all dream of the Earp brothers and Doc cleaning up the riff raft and criminal element in our own towns and even Washington D.C.?
You're right about Powers Booth's performance, he was outstanding, in fact everyone acting in the film was. My wife normally has little interest in Western films but she loved Tombstone. It is just great entertainment with some of the most memorable, and quotable, lines in movie history.
For me, the best shot for good horsemanship was Ike Clanton riding off after dropping his red sash on the ground. Man, what a rider. Kurt Russell and Sam Elliott always ride nicely ... Val Kilmer's horsemanship looked like he was a little green.
Off subject ... my favorite movie horseman ever ... well, two ... Yakima Canut and Ben Johnson. With Ben Johnson, he just thinks about going over there or halting here, and the horse does it. I love watching him on horseback.
Everybody was tops. Michael Biehn as screwy Johnny Ringo made the movie in a lot of ways. It had so many perfect casting matches. In this house, we thought it was just us that could watch the stupid thing over and over, felt compelled to even, and loved it every time. Still do a couple times a year. Then we find out that it's not just us, it's a lot of people, all different kinds. A friend's teen-aged daughter must have watched it a dozen times.
Just one of those stars-aligned-right movies, I guess.
***You’re right about Powers Booth’s performance, he was outstanding, in fact everyone acting in the film was.****
Powers Booth did a great imitation of Lee Marvin in that film.