I'd heard reports that the servers were all very very low pop, but i was watching a friend who reupped his sub because he wanted too see whats up, and they aren't lying. At prime time on a friday this game was absolutetly dead, everything was listed as low pop and i only saw 2 other people in gotham as he flew about.
But, having seen many an mmo go through this, i'd like to offer my perspective on what happened.
1. Samey content.
Almost every quest was identical. It wasn't Star Wars Online bad, but it was very noticable.
2. Nothing you did mattered. This is fricken unforgiveable in a post Cataclysm world. A good example of this is a very early quest, where you are called and told Zatana is in trouble, go help her! I got this quest while standing about 5 feet from Zatana in the police station. Almost every quest chain ends with you fighting a hero/villian, then getting a call from a hero saying good job. There's no sense you did anything.
Cataclysm showed that you can give an illusion of change in an mmo, and given that you are a friggen super hero, it would have been nice too feel epic once or twice.
3. Not enough content in general. Yes, the game just came out, but given how fast leveling was, they should have prepared for this.
4. The areas are boring. Gotham and Metropolis look really nice from the air or the rooftops, but when you get on the street they are all either barren wastelands or insane quest areas. They don't feel like real places. They are big, but compared to many mmos, they have a much smaller gameworld, because of this, they really should have spent some time in making them feel like cities.
5. Bad explanation of game mechanics, and possibly too simple in gameplay. I complaigned about this during beta. They don't take the time to explain the roles and the powers too people. You have to go onto the forums and read players who took the time to theorycraft the stuff, including what stats you would want, because the game didn't do a good job of telling you.
And honestly, in the end, no matter your build, you didn't have a lot of variety, and there is little to no skill involved in playing. You mash buttons and that's about it, it doesn't even matter what order you hit the buttons in or what you are fighting, you just clicked the button. It was like playing a more interactive Diablo. Lack of depth kills an mmo in the longterm, because you can't get better, you just get better gear. No changes to the class would change the way a person plays.
I've got other complaints, but i think these are some big reasons why it didn't work.
Ironicly, i think it has a lot of the same problems City of Heroes had, but City of Heroes, while never a huge success, maintained a decent population for many years, even today (decent as in enough people play and there are few enough servers that the world didn't seem dead).