Whether or not being slow to make his mind constitutes a weakness on Treebeard's part is debatable, but he certainly has a sympathy for living, growing things, which works as a strength and a weakness - it contributed to the Ents' decision to destroy Isengard, and which Saruman later manipulated to escape Orthanc. Thus it seems to be a crucial part of both his character - as the protection of living plants is his occupation - and also of the story as a whole. Our strengths are our weaknesses, I suppose.
