Obviously there's some subjectivity involved in these discussions but in Serena's case you have a situation where direct contemporaries who were her rivals retired early, and retired often. People who gave her trouble, particularly Henin but also Clijsters and others are all retired/mothers/coaches/etc. now. Navratilova and Evert have two less slams than she does, but they were challenging each other in final after final. Who's Serena's biggest challenger these days? Sharapova, who hasn't won a match against her in over a decade? Azarenka maybe? Cornet if she draws Serena early again in a slam? Give me a break.
Of course, Serena's longevity is in and of itself is a sign of greatness, but she had long stretches of time where she didn't dominate (her first year-end #1 was in 2002, and then the second was in 2009), whether it was due to injury or personal issues or the dominance of others (mainly Henin from 2006-early 2008). When Safina was #1 without a slam Serena chose to talk shit about Dinara instead of working on herself and getting her own ranking up to where it should be.
In contrast, Roger had long stretches of dominance lasting not for a season but for YEARS. Over four YEARS at #1, whereas Serena only passed Henin's total number of weeks at #1 a couple years ago even though by that time Henin was already twice retired and hadn't been #1 in five years. For Roger you have five slams in a row at Wimbledon, five in a row at the USO, etc. Finals of all four slams in 2006/2007/2009. He won his last five slams in the period from 2008 and onward, when Rafole broke out and began to dominate. And never once did he talk shit about other #1s being undeserving of their position. It's only in the last few years, now that Serena's contemporaries are well and truly gone, that she has begun to set up a truly lasting and durable period of dominance. People have been talking about how she's become so much more mellow and sporting these days: sure, pretty easy to do when you're at the top and no one is a real or lasting threat to you.
It's an indictment of the younger generations that have come after that no one seems to be able to challenge Serena for any sustained period of time. Even Kim has said she'd be confident that she could still win titles and slams if she played these days, it's just that she doesn't want to go through the grind of the tour, physical training, rehab, etc. This fact - that a huge swath of the best of the women's tour has retired early - is pretty obvious to most who pay attention to the WTA.
Serena is clearly a great, but comparing her to Roger is ridiculous.
The thing that prevented Serena from being number 1 longer is her scheduling, she played before like 10 tournaments a year. Second, How is Serena's domination any different than Federer's long reign, Federer didn't have much competition outside of clay, Nadal was mediocre on other surfaces. It's like when Serena dominates there is no competition, When Federer dominates he is really good. Also in 2007, Serena was just coming back from injury and the loss of her sister, Henin was lucky to beat her in Wimbledon and US Open. Yes Henin is better at clay than Serena, but Henin's head to head with Serena was mainly because Serena wasnt on form yet.
Also, why keep bringing up CLijsters, she is 2-7 against Serena, i wouldnt call that record a rival. People bash Azarenka for not being good enough, only close but no cigar, Clijsters is the same with Serena. Also, with the Safina thing, Roger did the same bashing to Nadal, Djokovic and Murray when they were beating him, but i dont hear any comments from you.