As far as the question of narrative and character -- I don't know this for a fact, but I think that Robbins was responding to the expectations that many people had of him based on his musical theater work. Compared to "West Side Story" or "Fiddler on the Roof," "Dances at a Gathering" doesn't "tell a story," but it does evoke a series of moods and suggest a wild variety of people.
This is an important point, since Dances marked Robbins' return to the ballet stage after swearing off verbal theater. He probably wanted a clean distinction between the genres.
I've never seen ..."Watermill",
I respectfully disagree. The first time I saw WM (with Villella, which is significant), it changed my life. It is entirely dependent upon the performer and on the viewer's ability (which, alas, I don't always have) to become fully immersed in the very slow action on stage.
Would I want to see it every month? No! No more than once every four to five years and only with a dancer able to convey the extreme intensity necessary to make it work. But I consider it one of Robbins' best works.
La Rocco lost me with this:
'Robbins is often more interesting when he isn't trying to be so serious; such is the case with "West Side Story Suite." '
LaRocco chose a poor example in citing WSSS, which (except for a new dance for Tony) was not intended as ballet. It clearly stems from his desire to preserve what many (including himself?) consider his greatest work. A better example of a less serious effort might be The Four Seasons, a gentle spoof of ballet genres.