The binary nature of mez in CoH has always been one of its weakest features. It's also forced some major unintended design changes on the game.
When the game launched, it only had one mechanism that would help you if you were hit by a mezzing attack - mez protection. This is what Tankers and Scrappers have. Under this mechanism, you have a mez threshold which an attacker has to exceed in order to mez you. It's still very binary, but the values are high enough that if you have Tank or Scrapper type mez protection, you probably don't have to worry much about mez in practice. I played from open beta to the end of 2008 and I don't think I ever had a toggled-up Tank mezzed in all that time.
There were no Break-Free inspirations. There was a mez protection insp, but you had to pop it before you were mezzed - no power, including insps, could activate when you were mezzed. Mez also dropped all running toggles.
The fact that you were de-toggled was actually a real problem for Tanks and Scrappers, ironically. At launch, the game only allowed you to run one toggle at a time (or was it one defensive toggle? I forget) and past a certain relatively low level, Tankers found they couldn't do their job if they didn't run their mez protection toggle 24/7. If you tried running a damage resistance toggle instead then you jumped into a spawn, got held/slept/stunned, lost your damage resistance toggle anyway and were unable to protect the team. This left them with sets full of toggles they couldn't use at mid to high levels.
This led directly to Issue 3, which fixed the issue by allowing you to run as many toggles as you wanted to. Unfortunately that introduced a slew of other problems, because there were no damage resistance caps in those days. Certain Tanker sets, in combination with the Fighting pool, could actually give you >100% smashing/lethal resistance. The devs responded by introducing resistance and defence caps, then noted that Tankers could still hit the caps "too easily" and a round of nerfs to defensive powers began. Pool powers, which were initially intended to provide some protection for Blasters, Defenders and Controllers, were nerfed hardest, leading to a counter-intuitive situation where they were little use to those AT's and of most use to Tankers, who could still use them to hit or get close to their S/L caps.
Even powers like Stealth used to give fairly significant chunks of defence. I'm going by dim and distant memory, but a figure of 7.5% springs to mind (half that if you broke stealth by attacking). Given that ED wasn't around at that time, you could triple that value if you 6-slotted for defence. In the post-ED, post-IO world, the figures such powers offer are derisory by comparison.
Issue 3 was also the point where Stamina stopped being a nice-to-have. All those toggles could tire a body fast.
It's a pity, because I actually rather liked that earlier design - or I would have done, if it had worked. Tankers and Scrappers required a bit more thought to play in a know-your-enemy kind of a way, but because you only ran one toggle at a time, you theoretically had a lot more build freedom. Meanwhile, the squishier AT's had ways to get reasonable, but not ridiculous defences if they needed them, by dipping into the Fighting ot Stealth pools.
It didn't work though. IMO they should just have made the Tank and Scrapper mez protection toggles into autos instead, but they wanted to keep the possibility of a Tank being de-toggled by enough mez. Like I said though, I never saw it happen.
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That's all ancient history. The game now has other mechanisms for dealing with Mez.
Break-Frees show that the engine allows for click powers that can activate when mezzed. Blaster T1 & T2 attacks also.
Mez resistance is now a thing. This shortens the duration of mez, but doesn't change your mez threshold.
Toggles which don't drop when you're mezzed are now possible.
Metabolic Acceleration from the Atomic Manipulation set is a great example, being a mez resistance toggle that doesn't drop when you're actually stunned, slept or held. It would be pretty useless if it did, when you think about it.
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If I was getting radical and wanted to make the mez situation less of an on-off switch than it currently is, I might do something like the following.
Make mez resistance much more common across all AT's. Put it into autos, or into toggles and clicks that ignore mez.
Make those same powers enhanceable for mez resistance.
Give every character an inherent Break-Free power of short duration. Slottable for recharge, but by no means perma-able.
Lower, but do not remove the mez protection level of those powers that currently offer it.
If mechanically possible, set a separate, higher and AT-dependent threshold for dropping toggles.
That creates a game where every character has some form of defence against mez, but also one where mez doesn't necessarily leave you completely helpless. Enough mez will de-toggle you, but that first hold or stun won't necessarily mean game-over. Enhanceable mez resistance means that it's harder to mobs to overlap holds, giving you at least some freedom of action, and the inherent Break-Free gives you an escape option.
If you play a Tanker, Brute, Stalker or Scrapper you might actually end up briefly mezzed from time to time, but fully dropping your toggles would be as hard as it is right now (e.g. very hard indeed).