One person said this to me. Champions online is very wide, true, but it's only an inch deep. City of heroes wasn't just wide, it had multiple layers of soil, sedimentary layers, igneous intrusive, igneous extrusive, and metamorphic layers. It took time to discover that and it was very satisfying to. Both games were easy to learn but, city of heroes kept you learning well after the tutorial. Champions online that learning stops short.
Its really difficult to explain from a design perspective, but to me the best games have a balance between freedom and structure. There must be enough freedom to be interesting, but that freedom has to exist within an environment of structure or its often pointless. The best analogy I can come up with is painting with black ink on a white canvas. You can only use so much black ink on the white canvas before it becomes a black canvas with a white drawing. At some point, removing too much structure makes the freedom that remains an amorphous blob. I think the City of Heroes powers and archetype system had a good amount of freedom and structure - not perfect, but good. CO removes too much structure, and you end up with only a little bit of interesting white in a sea of black. There are more possible combinations of builds in CO, but less interesting combinations.
The *engine* is in many ways far superior. But its used in a way that blurs so many distinctions between how things work that it actually eliminates some of the choices that CoH has. The melee landscape was very rich in CoH specifically because the archetype structure made different choices interestingly distinct. The melee landscape in CO is a lot blander, even if superficially there are more options in some areas. The buff and control landscape was far superior in CoH, even if it was a little broken. By being relatively straight forward, the gear system in CoH was more interesting to explore: its a larger, but in my opinion far more confused mess in CO. There are no easy obvious paths to guide the novice, and optimization is very opaque unless you are an expert.
I wouldn't say there's nothing to learn past the tutorial. I would say there's no obvious roadmap to anything past the tutorial. A CoH player could start with the standard enhancements and graduate to common IOs and then begin experimenting with sets in an organic way. And its obvious you are looking at three different systems of enhancement, each with advantages over the previous in stages. You're introduced to them gradually, and you can safely ignore almost all of it without hurting yourself significantly.
If you jump into CO now, starting from scratch, you have to contend with having a primary and secondary offense slot you can put gear in, without any guidance as to what might be good or bad to put in those. And you only get one each, so you have to make choices immediately. But from day one you'll be confronted with gear that increases stats, gear that provides powers, gear that modifies powers, and no way to judge what might be better or worse except maybe via the cost of the items - which is *not* always a good guide (and of course just to make life a little more difficult instead of one linear currency value they use a denominated currency, because why not).
So you have six different slots (primary/secondary offense/defense/utility) and yet all of the gear that goes into each slot can theoretically offer stat boosts, offensive or defensive abilities, and offensive or defensive boosts. That's an example of what I call "worthless detail." Six different slots, but really six slots with arbitrary distinctions on what you can slot in them. Right now one of my CO alts has offensive gear that buffs my defense stat and defensive gear that buffs offense, because of course they do. Its a level of detail that is confusing, but in no way creates any opportunity for more interesting builds than would be possible if all six were generic slots. In CoH there were set inventions limited to different kinds of powers, and while the bonuses they offered were not always logical, the fact that different archetypes were likely to benefit more from certain sets (by having more of those kinds of powers) meant those differences had an actual impact on build diversity. A case where structure highlights differences, and lack of structure erases them, making the lack of structure ironically remove choices.