When regarding Borg and the players of that era then the fact that nobody really played the Australian does need to be taken into account. This and the whole amateur/pro divide obviously means that simply counting Majors is insufficient.
Borg's retired young because he was burnt-out. If he had continued to play then it is hardly a given he would've won more, due to this. Keep in mind that he had won Majors for 8 years in-a-row at this stage, which is the same as Sampras and Federer. It isn't as if he would be expected to have another 3-4 years of Major wins in him.
Borg's phenomenal level on courts of all speeds is what I find most impressive. 5 consecutive Wimbledon titles on fast, low-bouncing grass and 6 wins on the slow, high-bouncing clay of Roland Garros. 3 consecutive French Open-Wimbledon doubles is one of the all-time great achievements. His weakest Major (considering he played the Australian only once) was really the US Open with 4 finals, losing to two other greats McEnroe and Connors twice each. He only ever played 4 Majors on hard and reached the final of 3 of them. By comparison, it took Federer and Nadal until their 11th Major on clay and hard respectively to win one and Sampras played 13 Majors on clay, reaching only one semi. His 22 titles on carpet also show his ability on fast courts. He had 3 years where he reached 3 Majors finals out of 3 played. The only reason his number of weeks at #1 is so low is due to the poor system. He was voted ATP Player of the Year for 3 years that he did not end as #1.
Due to Sampras' relatively poor showings on clay, I place Borg ahead of him, but behind Federer in my ranking of the greatest players of the Open era. One of the very, very best.