OK finally I think I would like to respond to LP, because he has always made the most clear and articulate case that CSM is OP.
First, put aside religion swaps, because as I have already said, these only hurt you if you foolishly only spread one religion in your cities instead of spreading as many as possible. It is not possible that the enemy could switch you religion unless you already have the religion in one of your cities, and if you have the religion in one then you can spread it to all, essentially negating (or at least substantialy nerfing) the religion swap. So lets focus only on civic switches.
Now it is unlikely that a Powerful Nation would CSM another Powerful nation. Why? Because first, as a powerful nation, you are running good civics. You must switch to a crappy civic in order to switch your opponent to something crappy. That means lost hammers/gold etc to anarchy. And as long as you want to keep them in the Crappy civic, you must stay in it too. So you are harmed just as much as they are, because you have to stay in Despotism too. But it is worse for you, because in addition, you had to lose a turn to Anarchy to get in the crap civic in the first place while they did not. CSM switches their civic with NO ANARCHY. As long as they do not foolishly try to switch back to the old Civic, until you switch first, you end up losing more than they do. So one Superpower is not going to CSM another Superpower for a net loss. It just does not make sense. Plus a big powerful Civ will have the GNP to match you in espionage, and run Counterespionage on you, so it will be almost impossible to keep them pinned in crap civics while still teching, etc, its just unsustainably expensive.
CSM is generally going to be used by a Strong Civ against a small, weak one. Now in this scenario, HOW the bigger civ beats the small is irrelevant. Big beats small, period. They beat him because they have more... More money, more culture, more population, more tech, more espy. Whatever they have more of, they use it to destroy the weaker civ. It doesent matter if they destroy him with CSM or with Tanks, he was small and weak and he was going down and no one could stop it. So CSM by strong against weak is a non-issue. That brings me to the most interesting part... LP's argument:
The main problem comes when you have some very large and powerful nations, and some relatively small and weak ones. The tiny civs have no real chance to win anyway, so lose little to nothing by switching to crappy civics themselves. They then have an incredibly disproportionate influence over the game by choosing which large nation's economy to throw into complete disarray
Now 2metra had the correct response to this:
Well, who will protect this small and weak nation from the wrath of the powerful nation who is very angry at them? Not to mention that small and weak nation will have small and weak economy and will have hard times keeping espionage to out-espionage the big and powerful nation.
In other words, the solution for the Big Civ is simple, just invade and crush the small one if they have the audacity to CSM you. He also correctly points out that a small civ cant keep generating the hundreds of EP needed to run CSM, especially against a Big Civ who can run jacked up espy percentages on them and has the EP to sustain Counterespionage missions. LP has an interesting response, on how the little civ gets away with it:
Whichever larger nation they align themselves with. If the smaller nation is acting alone then sure, they won't last long - but usually that will not be the case.
Which means that in order for little civ to pull this off he must be the
ally of another Superpower (this has been described by some here as "
collusion")
Which brings us to the point. If little-civ1 has to
"collude" With Big-Civ1 in order to use CSM on Big-Civ2 effectively, then surely its no great stretch of the imagination that Big-Civ2 would also
"collude" with his own ally, little-civ2 to instantly use the CSM to switch him right back into the civics he was already in.
What this means, is that CSM will not be the non-stop neverending tactic some people claim, but instead it will be used as intended, in key strategic situations, like to stop someone from getting a tech or finishing a Wonder or to spread religion or a Corporation, or something like that.