I think a lot of what leads to the 'CO is too twitchy' bit is that things just happen faster. In part because of a lot of different things such as:
- The minion equivalent isn't actually, CO henchmen are closer to CoH underlings in capability and more importantly durability. They die much faster, and if you use that as a base rather than henchmen = minions it actually works out all the way up to Cosmics being Giant Monsters. Everything is a step down from what it was in CoH, effectively. Which makes combat move faster because things die faster.
- Player power level is a lot lower, as well. In CoH, even a blaster could stand up to a few hits here and there and be fine. In CO, if you build a pure offense blaster style character you'll literally get knocked over by a stiff breeze. And CO tanks are more comparable to CoH scrappers or brutes in defensive capability, plus there's no ridiculously awesome buffs/debuffs in CO to make up the slack.
- Ranged vs melee balance is significantly different to how it was in CoH. In CoH, melee powers in general did somewhat more damage than comparable ranged powers. This was justified by devs with range being a defense, but the real truth of that aside it affected something else. Fights tended to clump up around the melee characters because in general the melee characters were both the ones doing the most damage and thus the highest threat, and also the only tanking ATs were all melee. IOed tanking defenders aside, that is. Meanwhile in CO anyone can be a tank, every set has the taunt advantages on some power or other. Additionally melee/range damage is skewed the other way, range tends to do more damage in CO. Which combined with the ranged tanks mean mobs go in all directions which makes for a much more mobile fight.
- Blocking. The whole mechanic of having special attacks that must be blocked is cool in theory, but in my opinion falls apart when there's five or six mobs on screen. But that's irrelevent for the purposes of this discussion, what's relevent here is when one of those special attacks happens you have to block in time or you just eat it. Which requires more attention paid.
- In CoH, you hit a button and the power went off after some length of animation time. In CO, you have to hold the button to charge or maintain a power. Or tap it constantly for tap and combo powers. Or make an autohotkey script to hit the button every half a second while it's held down to spam it constantly without wearing out your keyboard but that's another issue entirely. This means fingers have to be on buttons a lot more often and a lot higher percentage of time.
- No power queueing. You can't hit another power to fire it immediately after the current one's done in CO like you could in CoH. So again, more attention required.
Any one of those on its own probably wouldn't be that bad. But all of them together mean you have to be paying attention in combat much more in CO than you ever did in CoH. In CoH, even in incarnate trials I could stop in the middle of a fight to type a quick message in chat and generally be fine. In CO I can't because mobs move away or I get nailed with something I was supposed to block and I have to be constantly holding buttons to do anything while in CoH I could activate one power and queue up another and have 4-5 seconds to type before I had to do anything else. I know some people would consider that a bad thing but honestly it ranks among the best features of CoH for me, that one thing evolved from a heap of little details. Mostly because I RP (Yeah, I know I'm weird) and in CoH I could keep up RP and combat at the same time without missing much on either side. In CO I can't, because my attention has to constantly be on the fight.
