I will really miss CoH, it was the only MMO that I've stuck with for its entire life. Sure, I've stopped and restarted playing a few times over the years, but CoH was beautiful in the fact that you could do that and come back and not be totally left behind in gear to the point where you couldn't play with your friends. CoH is the only MMO that I ever came back to after I stopped playing it for a few months. Their exemplar and sidekick systems were excellent and allowed you to play easily with your low level or high level friends and allowed both the casuals and more hardcore players to play together. Their merits system made it worthwhile to go back and do the old task forces so low level content wasn't ignored once it was out-leveled.
I really liked the IO system that came out because it allowed you to, when combined with powerset selection, make your characters behave very much how you wanted them to or to fix inherent weaknesses of each powerset such as 2 ft walls vs stone tanks or knockdown vs fire tanks (rather than every Tank class being more or less copies of each other like in most MMOs). This, along with a phenomenal character creator, allowed for just awesome levels of character customization that allowed you to create characters that were truly your own. It was rare, unless you took a forums cookie-cutter build, to run into someone else who played and looked exactly like you did, unlike most other MMOs where everyone looks related and has the same powers. I've yet to play any other MMOs that had this level of choice, the skill-tree based MMOs seem to offer a lot of choice, but unfortunately most of them are so poorly balanced it isn't long before there's only a few acceptable builds for each task (dps, tank, healer).
Finally, I really liked the combat system, it was so much faster paced than the standard MMO combat of pull, beat on mob for 30-60 seconds, pull again. Coming from EQ when I started CoH it was a very welcome change from the Tab-auto-attack system of play (or as a wizard, stand up, cast 1 spell, sit down). In a sea of EQ-clones, WoW and WoW-clones, this active, fast-paced, combat was very enjoyable to me I truly felt powerful playing my character and that my choices and actions more greatly affected the outcome than the MMO's RNG (which is probably why I enjoy Tera now, which is admittedly a niche MMO as well). Similar to the combat system, the buffs were so very powerful and gameplay changing, a Force Field bubbler could make your team nigh-indestructible, a kinetics would turn your team into a damage powerhouse with speed boost and fulcrum shift, again a welcome change from EQ where the very best cleric buffs maybe reduced incoming damage a few percent (compared to force field, once fully slotted, making it so the mob missed 90-95% of the time).