Interesting that some praise the score but are critical of the book, referring to it as "clunky". If I recall, Marsha Norman won a Tony for "Best Book of a Musical".
I think it is unfortunate to drop the songs "Round Shouldered Man" and "Race You To the Top of the Morning" as they are both lovely songs and they flesh out the relationship between Colin and his father, which is pivotal to the plot.
I am also surprised to hear the show described as a "soaring emotional aria-filled, large production musical" as I always thought it was one of the smaller, less bombastic musicals of its time. Remember, it opened the same year as "Miss Saigon" which sounds more like the type of musical you are describing. By comparison, TSG felt more like a chamber musical.
I saw the original Broadway production many times (at least ten) and was there for the final performance, so obviously I am a fan. It is a show that can be a very dark, somber evening at the theater without the right touch. The original creators were able to find a lot of humor in the piece, especially in Mary's and Colin's bratty behavior. I also remember that the final scene, where the lights come up on the garden in full bloom for the first time, always got a big round of applause for the set designer (Heidi Landesman now Ettinger).. And I still recall one of the most moving moments ever, when Lily and Colin end "Come To My Garden/Lift Me up" and the lights slowly go down around them until there is just one pinpoint of light on Colin as he falls asleep in his mother's arms. It really was an exquisite production, very delicately handled for the most part. I think a revival would need a very special director and design team who could really wow us with their concept. Not sure if this DC production meets those standards, but I'm just judging from pictures and video clips, so I could be wrong.