While Mega Salamence is powerful and possibly the best offensive Pokemon in the tier with a lot of different sets (although the most common is DD), it's not impossible to counter. Not at all.
For example:
Rotom-W and Rotom-H can burn it and completely wall it.
Suicune with Ice Beam with 172 EVs and Modest can survive a 252 Double-Edge at +1 and always OHKO with Ice Beam. I've also seen Tailwind be more prominent in ORAS Doubles, which, by the way Suicune learns, this makes Megamence a lot easier to stop.
172+ SpA Suicune Ice Beam vs. 4 HP / 0 SpD Salamence: 376-444 (113.2 - 133.7%) -- guaranteed OHKO
+1 252 Atk Aerilate Salamence Double-Edge vs. 252 HP / 84 Def Suicune: 340-402 (84.1 - 99.5%) -- guaranteed 2HKO after Leftovers recovery
This set/EV spread is not bad at all, try it.
There are also mons that can use Follow Me (Jirachi, Togekiss) to help get rid of Mence. Another thing is Trick Room, which not only Mega Salamence has a hard time dealing with, most teams are completely unable to deal with it.
Mega Salamence is versatile, and a big threat with it's amazing stats but it is far from impossible to stop. So while it can take out entire teams if they lack counters, it has a lot of weaknesses, suffers from 4MSS and it doesn't have the best SpD - it is not that far from being banworthy, but not really that big of an issue.
""tl;dr"": I am against banning Mega Salamence as it is feasible to counter if prepared well enough.
This right here is actually giving me somewhat of a change of heart on Mega Mence itself. It can muscle by most of its supposed checks, but the Rotoms are a full stop. Going special won't help either, given that most Rotom invest in SpD. That Suicune spread is also very creative, and is effective outside of beating Mega Mence as well. Also, thanks for bringing up Tailwind and Trick Room, as they are are becoming more prominent for the sake of outspeeding the new fast Megas. I was mistaken in saying that the rise of those moves rests on Mega Salamence- it also rests on Gallade, Diancie, and Metagross, among others. It's not Mence overcentralizing the metagame, it's a trend.
Key word in the above paragraph is "somewhat", though. Even some Pokemon that have gotten the banhammer had viable checks and counters, such as Bisharp being a great answer to Aegislash, or Fire-types being great answers to Genesect. Just because a Pokemon has hard counters doesn't necessarily mean it isn't broken. As has been stated back on page 1, Mega Mence may have counters, but it has plenty of ways to deal with its checks despite 4MSS. Couple that with its raw power that we all know, and you've got a monster on your hands even if there's a Rotom or TR setter waiting in the wings.
Moreover, remember that Mega Mence always has a partner alongside it. On page 2, you'll find everything you need to know about type synergy and team support. It's not that difficult to provide redirection support from partners like Jirachi, or partner Mence with something that can defeat its counters, like Skymin against Rotom-W.
252 SpA Life Orb Shaymin-S Seed Flare vs. 252 HP / 252+ SpD Rotom-W: 330-393 (108.5 - 129.2%) -- guaranteed OHKO
In this case, you'd better have another Mence counter to switch to. With all of the above in mind, the fact that Mega Salamence synergizes so well with many Pokemon means it nearly always has plenty of opportunities to sweep an opponent. It's already very good as a standalone mon, and with proper teambuilding, it has the potential to become well-nigh unstoppable.
TL;DR version: Mega Mence's viable counters in the Rotoms and Tailwind, among other things, means it won't always get the job done on its own, and the rise of the usage of said counters is not solely for Mence. Nonetheless, it's a great Pokemon who can mesh with many teammates to effectively deal with such counters, which arguably makes it too much to handle for most teams. Clefairy, you've definitely swayed my opinion a bit, but I'm still leaning slightly towards the
ban side. But hey, I could go either way at this point.
Ok I've been playing around on the two ladders and I find that I like the suspect one much better. I admit it's a little because i just don't like dealing with things but the teams i make for a suspect ladder don't have to constantly worry about the fact that they need 1-2 mons specifically for beating this thing. Also things like mega venusaur are more usable and add a little more mixture to the tier other then mence teams and anti mence teams.
Tbh, the ladders aren't that different. Like Haruno, I have seen frighteningly little Mega Mence teams on the regular ladder. Still, like you, I have seen many more anti-Mence teams there (think: Trick Room). Perhaps it's a result of the ladder overcompensating.