I was subscribed to the game for several months after CoH went down.
I found CO's power system very limiting concept-wise. There was only one fire powerset, there was only one electric powerset, etc. But CoH has different powersets for each AT, so I can make an Elec/Elec Brute an Elec/Elec Blaster, or an Elec/Elec Dominator and they won't play even remotely the same.
Related to that, I hated the way that "crowd control" means "mind control, period" rather than the huge variety of options for a Controller or Dominator on CoH, and hated even more that using the same Hold on an enemy caused them to resist it more easily rather than stacking the effect.
I wasn't fond of the gear system. I don't understand why I can't just make my "Invulnerability" power stronger directly, rather than putting on a new pair of shoes that somehow protects my face. (At least the gear is invisible, unlike DCUO)
I hated how weak most melee attacks felt. Just a random flailing of limbs with no weight behind the blows. My martial arts hero looked like a Looney Tunes character.
I hated that status protection was tied to perfectly-timed blocks that I had to somehow fit between all that limb flailing (even moreso when there's a bit of lag and the block command doesn't actually go through until way too late).
I hated only being able to have one defensive passive active at a time.
I hated that damage resistance used a "diminishing returns" system. I hated that my characters felt so paper-thin, even "invulnerable" ones (since I needed 900% damage resist on CO to match my Elec/Invuln Brute's 90%, and that was impossible).
And even had I gotten that 900%, it still would have been weaker than my CoH character, because my CoH character also had defense and health regen on top of the damage resistance, while on CO, you can have only one of those (and "dodging" in CO was also just damage resistance rather than actually avoiding the attack).
I hated how samey most characters felt at low level. A level 1 Brute and a Level 1 Controller don't play anywhere near the same, but ever level 1 CO character plays pretty much like every other level 1 CO character; the only difference is how close you have to be to do damage.
I hated that all of the preceding factors made me feel like I was never in control of combat the way I was in CoH.
I hated that mission completion accounted for most of your XP, making street-sweeping all but worthless.
I don't understand why, when I took a flight power, the default flight angle was about 30° downwards rather than level, and nobody could explain it to me. I had to be constantly readjusting my camera angle to fly straight.
I posted once on the CO forums that I wished there wasn't a "best" power for every slot so I have some choice, and I just got people going "a choice between a crappy power and a good power is still a choice!" When I said "No, I'd really like a choice between equally-good powers", they just started replying with pictures of laughing ponies as if I'd asked for the moon. So the community was crap, too.
(I use the past tense because I haven't played since 2013. Have they fixed any of those issues in the past six years?)
There were many things I liked about the game conceptually, but it just felt like a slog to play. I never felt like a superhero. I wasn't having fun. The only character I enjoyed was my pseudo-Mastermind with all the pets, and even he was a slog at times.