Both games are based on the superhero genre, yes.  But COH is an MMO first and for most. It is not designed like a traditional RPG game.  THERE are no stats in this game.  None whatsoever.  Granted your choice of archetype affect how much hits you have, but beyond that there is no trace of the attributes that permeate ever game system.  Even the simplest have a few -- Strength, Dexterity, Intelligence, etc., even if they are called something else. This harkens back to the grand-daddy system of Dungeons and Dragons - which was the first.  Now other systems are more streamlined.  It depends on what they are trying to simulate and how detailed they want to be versus how fast they want to play.  Its a balancing act.

Now if you came into City of Heroes expecting an RPG, you may not like it.  I've seen more than a few people try several character types, but never play beyond a handful of levels.  The game will seem repetitive if you don't pass certain thresholds -- level 14 (travel power), level 20 (New zones), level 30 (More new zones, auras), level 40(elite power sets), etc.  Not getting into a good SG, wether it is RP or not, also will have an effect on your enjoyment and progress of the game.  Personally having a non-game based RP event is something the designers don't do that is all up to the players.  You can't beat that.  Whether you are a bunch of school kids traveling together, armored future warriors, or hi-tech police officers, a good theme can change the game.

Now from what I've heard , Champions Online is heavily based on the RPG that it gains it name from.  I've not played the online game yet, but I've played the paper RPG game.  That means you WILL have to think about whether you are making a fast hero, a strong hero, a tough hero from the get-go.  If you came from City of Heroes and aren't a paper RPG player, you may find some of these concepts daunting.  Whether this will make the game hard to get into, I don't know.  I have heard that forming a SG is A LOT harder to do than in City.  You have to have a whole team together and ready.  Finding time when ALL would-be officers are on at the same time just to activate a supergroup seems a little daunting and could really reduce the number of such groups.    I've also heard that teaming is not happening as much.  Lack of global communications as yet will also make communicating that a SG is available or a team is trying to set up quite difficult.  While City of Heroes has had 1 or 2 Atlas parks per server, CO could have dozens of Millennium Cities?

Even what I've heard about DC Universe online seems to suggest it isn't an MMO in every sense of the word.  Since it is partly hinged on the consoles, that may make sense there, but detract elsewhere.

The bottom line is that whether it is game mechanics, interface mechanics, server mechanics, role-playing mechanics will differ from game to game.  While City of Heroes might not be for everyone, the other games might not either.    

I did see a few bitter sentiments from those leaving vs. those staying.  I hope that we can find an equilibrium, and be heroes wherever suits us.  I hope the other games survive, but not at the expense of City. 

Time will tell who the victor will be.  I hope it us us.