More importantly than there being no counters to both the DD and Mixed sets is that there aren't any real counters to either. Both sets have that same Fire/Ground/Dragon type coverage that threatens everything in the game with a 2hko. Rotom may have something that can hurt nearly everything, but it isn't nearly as threatening. You don't have to worry about your Blissey or Tyranitar being 2hkod by Rotom, do you? You don't have to worry about your Machamp or Hitmontop or whatever being 2hkod by Tyranitar. Gengar and Lucario are the only remotely comparable pokemon in Standard to Salamence in that they both have multiple viable sets, excellent type coverage, and good resists. However, Lucario and Gengar are both much more frail and they have other issues to worry about, like having to use Focus Blast or Ice Punch/Stone Edge for coverage purposes or having a Pursuit weakness. Both of them have solid counters to what would in a vacuum be the best set (Gliscor/Salamence [lol] and Blissey/Scizor), and so they are forced to adapt to beat these natural counters. Mence has none of these problems. The only paper counter is Cresselia, which is terrible in the current metagame due to the lack of solid recovery in sand. Mence may have Stealth Rock to worry about (while the other two are essentially immune to one type of hazard), but Mence also has not one, but 2 reliable healing moves to compensate (yes, I suppose Wish is viable) while Lucario has nothing but rest and Gengar has the unreliable (though still good) Pain Split. For what it is worth Mence also doesn't take spikes damage unlike Lucario. For these reasons Salamence is offensively in a class of its own.
Another thing that differentiates Mence from the other 2 on the "Big 3" list is that it actually counters things. As has been mentioned, Mence ironically is one of the better Lucario counters available, while Lucario can only help revenge kill and Gengar has to hope that it has Switched in on the correct attack or it just dies (and Lucario is the only Offensive threat it has any real ability to counter on 3/4 moves), and even then it relies on shitty Focus Blast.
The Offensive and Defensive characteristics are too intangible and vague. We don't have a way to reliably define what constitutes enough of a portion of the metagame and how much damage it will do in a match. We don't have the correct statistics to reliably use them to decide quantitatively if a pokemon is Uber. I believe that we need an "Average Damage per match" statistic before these characteristics are of any value at all.
So, where does this leave us? We are forced to test a metagame without Mence to determine if the metagame is better off without him. It is very difficult to deny that Mence nearly always deserved a spot over other sweepers on Offense (I would say to a greater extent than Skarmory or Blissey on stall.) While it has helped make the metagame less stallish than it would normally, it also severely limits Offensive teams. When building offense it is rather obvious that Salamence is an excellent sweeper that also patches up any Lucario problems a team might have. When Mence is given a slot the team nearly builds itself afterwards. So, we have to ask ourselves, what is better? A metagame with more boring offense or a metagame with more stall and more varied offense? Honestly I think that I would enjoy the second metagame, because the power of Mence has caused people to become strategically lazy. People just throw a Mence onto a team and write off the stall matchup instead of taking the time to build a team that has real synergy. Now, I suppose that the same could be said about Breloom, Gengar, Lucario, Infernape, etc. regarding the stall matchup, but the differences between these pokemon and Mence have already been noted.
The best option is to ban Mence and once again reward good players for making good teams not just in tournaments but also on the ladder.
EDIT: Infernape probably deserves a spot in that top tier with Latias gone. Forgive my exclusion.