Well I typed out a really long reply for a diff. thread about "what killed CoH?" so...here's what was going to be in that....
But for a serious reply I'll chime in with a few thoughts.
* No "end-game" content...ie. nothing for lvl 50s to do. Back 4-5 years ago is when CoH was at the peak I guess...most of the people in my SG left for WoW. Why? Because there "was nothing to do" on their lvl 50 toon. Exemp and play with friends? That lasted a bit but...some people want/like to play 1 (or 2) toons and that's it. Doing Hamidon over and over (once it was found out how to defeat him) didn't satisfy them.
By the time we got raids/iTrials, it was too late, not enough people that really wanted to do them (and itrials came out too far and sparse).
*PvP. Yes, I'm one that never really cared for PvP. A bunch of people using SS/SJ and bouncing around kiting others and killing 'em. That's all I ever saw. Boooring (probably cause I was always playing a support toon which means I die first (targeted) in pvp fights

).
I've said it before that if they would have not done any PvP changes in i13 but added the ability to do the different builds for toons it might have been different. That way I wouldn't have to sacrifice my cool PvE build for some PvP action. I could do have my PvE and my PvP build and not have to worry (too much) about losing powers that I like in PvE.
I've also said there should be more PvE in PvP. Capture the flag, King of the Hill...something other than, "beat this guy up before he kills you." Recluse's Victory tried that I guess...but like one person said, they should have limited the zone to x players. So that after, let's say, 20 people are in RV, RV2 gets created.
Also needed better rewards for RV/pvp zones. Why should some random hero care if heroes are winning in RV? If he's not into pvp at all then he has no reason to "help the heroes!"
Maybe add some sort of buff (double xp, inf or ...who knows...something) that would last x amount of time for whatever side that 'won' in RV (or in a capture the flag type of thing).
I also like the idea that someone said about the whole, "do x faster than the villains (or heroes)." Like take out a building (saving it or burning it) and see who/which team can get the fastest time. *shrugs*
*Zone events. While yes, the Halloween (the one with the banners in the zones) was kind of cool...things like that should have been put in the back burner. Too many negatives for one positive (more shiny badges). They killed any missions in that zone where you had to hunt for mobs (*waves to Numina*) and they took too many people, usually, to complete; once the "badgers" got all of the badges, no one really did the events again.
No, adding a "do x event 5, 10, 15, x times" badge would not have been a good idea. Some other reward would have been nice (again maybe double xp or..who knows...something).
*Old content. Yes they were touching up stuff here and there but, again, "too little, too late." You could see the newer zones (and powersets/etc) being awesome and really finding out how to do a good zone. But with all that being said, the old zones were...ugh...blue side had too many of 'em and a few could have been touched up a long time ago.
Oh and Atlas Park should have been nuked and not Galaxy City! (EI's home was GC!)
I'll throw AE in to old content too instead of a separate topic. When it came out, and for the next month or so, it seemed that was the only thing going on. I log in and "everyone" was at AE (yes, not everyone...but most of them)...and not doing actual 'team' stuff... I know that had a bad taste in most players mouths that I played with.
*No real marketing for the game. It's been said to death but.....*shrugs* It seems like there could have been more done to market the game.
* VIP only server. I, at first, was glad that there was going to be a VIP only server when Freedom was going to launch but...thinking about it, I don't think there should have been a VIP only server at all. Just fractured the game/community more I think.
* Needed to work on cross-server teaming. iTrials are great (teaming in general) but when you can't get enough for a Lambda/MoM, etc....yeah...

This might have been nice for cross-server pvp'ing too.
* Lots of features that didn't get put in. Sure we were getting new features/content and 99% of the time it was great but...there are quite a few features that I would have loved get into the game.
For instance, inviting your own toons to your SG (or making the SG allow/invite your global toon/acct). Just one thing like that would have been nice (especially when "no one is on" to invite you to your own SG (granted free accounts help with this but still).
Overall, I think the 'death' is that we had such high hopes for a great MMO but the game was a "small budget" type of MMO...so we couldn't get the nice stuff (better graphics, etc...). Oh well...