Borg did have an amazing career, his versatility in order to adjust his game fron clay to grass was and still is unparalleled but he was never as dominant as other players great players have been, and that in my opinion is what excludes him from tier 1 greats (i consider him a top 5 player all time though behind Fed, Laver, Gonzalez and Sampras).
For example, during his prime years (1978-1981) he won 7 out of 11 majors played (64%). He also won 2 Masters. Federer in his prime years (2004-2007) won 11 out of 16 majors (69%) and in addition 3 Masters. Borg won a total of 34 titles whereas Federer won a total of 42 titles.
Overall not a big difference but nevertheless a clear one. I personally regard dominance in peak years as the main criteria to evaluate greatness and is my feeling that sometimes Borg is a bit overhyped around here as he was some superhuman being during his dominant years where it was almost impossible to beat him.