So, I would generally divide skills into three main types: technical (these are the ones I referred to as "tennis specific), strategic/mental (how prone to nerves is the player and how good at rethinking their strategy), and finally general athletic (how fit and strong is the player). In my view, both Seles and Del Potro are weakest in the third set, as neither is a great athlete. Sure, Del Potro doesn't have anything like the wide range of racket skills as the players you mentioned but he has very good groundstroke technique and can hit a forehand like almost nobody. Seles, also, had excellent groundstroke technique that combined power and accuracy, but she was rather slow, especially later in her career. By contrast, Graf was far quicker around the court. So, it seems to me that Seles and Del Potro fit what I said better than you suggested. For all that, though, we all know that our views of players are mostly non-rational and emotional and not the product of reasoning. We can after the fact attempt to justify them, but it's never the real reason. And, for me at least, I'll likely never again like a player as much as I liked the ones who I first knew of in my childhood.
For both Sampras and Federer, I agree that they are players I would ordinarily like, and I do quite like both of them - I would say I'm slightly more positive than neutral about both - but I find it really hard to cheer for someone who wins all the time. I was most fond of Seles after her stabbing.
Since you bring up Graf, I should note for the record that one of the reasons I am such a big fan of hers is that she is very much the Bull of the women's circuit. They are both exemplary sportspeople, who have delightful and modest demeanors, and handle their defeats with a dignity and grace that is very rarely seen. Not only are they exceptionally unwilling to engage in gamesmanship of any sort, but they also always refuse ever to admit that their few "defeats" were in fact moral victories caused by the cruel and unending list of injuries that unfairly afflicted both of them throughout their entire careers. It is only thanks to the stellar work of their fans that mere mortals such as you and I even know how debilitating both Graf and Bull's injuries were. For example, I used to post on a message board in which another poster was called Tomoaki. He once produced a long list of injuries that Graf had suffered that amounted to more than 100. However, Graf was so gracious in defeat that she had never once revealed any of them. I'm not quite sure how he uncovered this rare and precious information. Thankfully, many of Bull's fans have done similarly stellar work on his behalf, so we all know now that, much though Bull tries to hide it, he is in fact both unbeaten and unbeatable.
