In fairness, when I talked to other gamers about CoH while the game was still live, they always said that the reason they quit (or never bothered picking up the game) was that there was "nothing to do" once you hit the level cap. The people who played loved making alts (including me--I had 80 or so), but Positron had a point when he said that lack of endgame content was limiting their audience.
I don't think you'd want to go too far the other direction...WoW always feels to me like a never-ending treadmill of power creep...but I think that giving people things to do with their max-level characters was a good decision.
It's an interesting problem of design, really.
Assuming you've settled on a level-based advancement, with a level cap, your players have seen increases in power as they've progressed. Once they hit the level cap, you have two options (which are not exclusive):
1) Increase the level cap. This allows players to see their numbers climb higher, as they have come,to expect.
2) Broaden capability at the cap. This means giving more options for what an individual character can choose to do, but keeping those options at or around the power level of the level cap.
CoH chose (rightly, IMO) to go the second route. WoW (and most other MMOs) took the first.
Each option has its downside, though. Option 1 means lots and lots of new content, with bigger numbers, and has the effect of invalidating "gear" at the previous cap. Option 2 likely also means lots and lots of new content, but also means that the difficulty of said new content can't be hugely varied.
I believe that CoH was on the cusp of hitting a nice sweet spot: enough new powers that it took time to earn them all, and enough varied new content to use those powers in without getting bored of the same grindy content.
It wasn't there, yet (Praetorincarnate focus saw to that). But it was happening.
I personally think that finding the balance between new incarnate powers and new content needed more work - we had lots of powers, and relatively few places to use them.
But I do think it was happening.