I tried both CO and DCUO, in addition to many other MMOs after COH closed it's doors and honestly none of them ever caught on for me. I never did like CO's initial gear slotting and how you built out your powers, the goofy looking graphics, and just the lack of immersion with the game. With CoH I literally had a character I spent hours putting together, including a bio and building out their extra power pool and incarnate slots to fill out their character arc(and I did this with many characters) and I grew attached to my characters. I could never get that feeling with either CO or DCUO. Maybe it was just the fact I played COH for its entire existence and was just jaded to the other games, but they just never grabbed me. As an old pen and paper RPG player I just loved the amount of customization and input I had on my build. I loved trying new IO combinations, min maxing set bonuses to make my defender able to do amazing things solo'ing hard content, and just having so much flexibility with how I could build out my powers and character's overall performance. Were there builds and guides that were "the best" ? Sure...but that was also the fun thing about the game, you could make just about anything work.
I didn't even really touch on COH's best aspect, which was the community. I've never been a part of such an amazing community since this game shut down. I think around the time COH shut down, the gaming community overall was starting to become jaded and entitled, and toxicity and elitism started to become a real issue in a lot of MMOs, especially the ones that were F2P. Part of what made COH attractive to me early on was the paid subscription, which to me made people take the game more seriously and just in general created a better community aspect. People were paying a monthly fee to enjoy the game, so they were going to be sure to make sure it was enjoyable for themselves and every one else. Even when it went F2P eventually, the community remained amazing, while with games like WoW the community was falling apart and becoming completely toxic.