Can't really compare between different eras. These days, tennis has become much more commercial so the tournament directors has done what ever they can to increase the chances of the top players winning. They have included 32 seeds, slowing down of surfaces and there is a possibility of draw rigging. This applies to Federer and Djokovic as well.
You need to dominate more than one surface to be considered an all time great, imo. If Nadal can win a couple more wimbledons than I would consider him an all time great along with Federer, Borg, Sampras and Laver.