I visited this site constantly some time ago, but then it stopped being updated for some time. The owner has begun updating it again, and WoW's dominance has continued to grow. By subscriber base:

http://www.mmogchart.com/Chart1.html

You can see that not only has WoW continued to grow at a steady pace -- it clearly hasn't peaked and hasn't even slowed -- but also that all other MMOs on the market are actually shrinking, save Runescape and a few titles way down the subscriber chain. 

http://www.mmogchart.com/Chart7.html

Doing a quite mental count of what "all others" could consist of, I can confidently say that there are at least 30 games listed here, but it's very likely there are more. WoW's market share: 63 percent. Every other game in the genre: 37 percent. This market share is up from the same time last year, when it was closer to 55/45.

I have actually never played WoW since I know I would become addicted to it. What would you say is the secret to it's success? Basically what is it doing or has done that is so much better than every other MMORPG?

It's significantly more casual friendly. A great counter example would be Ultima Online: in that game, when you die, you literally lose all of your gear. If you work hours and hours to get the gear you want and then make a mistake, poof, everyone steals your stuff. Imagine playing Mass Effect for 15 hours, dying, and having to start over. Yes, that is literally how most MMOs worked in the earlier days.

Of course, if you play the game constantly, that's not a problem. You can get it back in a week or so. But if you only play once or twice a week? That's months of work, literally. So we had games like UO and Shadowbane, where the super hardcore players like myself absolutely loved it, because we ran over everybody else. Then we had the people who played... say, less than 30 hours a week, who hated it because they constantly got steamrolled by people like me and had to start all over again.

In WoW, there is absolutely no penalty for death. None whatsoever. When that was announced, it was some revolutionary concept in MMOs, but today it seems obvious. The short story: you don't have to play WoW 30 hours a week to feel good about your characters. For most other MMOs, you're either hardcore or you're dead.

I'm sure the word "casual" instantly puts you off, Legend, but trust me, based on what I know of you, you are absolutely correct that you'd get addicted. A casual MMO is still more "hardcore" than practically every other game out there. In WoW, you can play 1-2 hours in a sitting and be productive. Amazingly, this is a drastic improvement over other MMOs, where 4+ hours are usually necessary to accomplish goals on a daily basis. 

It is not entirely accurate to say there is no death penalty. There is a very very small penalty for your casual player, and a very severe penalty for your hardcore pve player. This works out really well I think because it keeps motivating the elite and does not penalize the casual. You could probably write an entire book on just how much better WoWs game design is compared to everything that came on the market before it.

Bleh I hate WoW because of the piss poor PvP. If I'm playing an MMORPG it will be a free RvR one.

Severe? :/ It's not severe at all. The death penalties involved in all cases are so pithy that I chose not even to acknowledge them rather than to take the 4-5 sentences needed to explain: "there is a tiny death penalty, but it's so comparatively small that it's not important." 

But regardless, Blizzard's singularly brilliant recognition was to not listen to MMO players. The people who yell loudly about how MMOs shoudl be done are people like me, who play literally 30-40-50 hours a week and want content suited for such unhealthy playstyles. They tuned these people out, and instead listened to the millions of PC gamers who were turned off by the horrendous amount of time needed to accomplish anything significant in an MMO. 

It really is a particularly powerful gift -- listening to the people who aren't talking because they aren't currently interested.  

I thought about getting City of Heroes/Villains because I like the theme, but haven't done it yet. Is it any good?

I want my gaming to be fun, not a chore. Just from second hand information, WoW seems like it can be a bit like doing chores sometimes, but that may be false.

If you want to be a strong character in most MMORPG's, you need to grind somewhat stranne.

In the sense that all MMOs are a chore, yes, it is, but City of Heroes/Villains is even worse, so I doubt that solves the problem.

Here's the core issue: designing content takes a whole lot longer than it takes to actually play through it. The base of WoW took 5 years to develop, but many players had hit level 60 and were in the original end-game dungeons within a month of the games release. 5 years to develop, 1 month to "beat." Or think of more traditional games; things like Super Mario Galaxy probably take a week for a "hardcore" player to beat, but it took years to develop. That's a problem.

So the solution must be to create repetitive content that can be churned out fast enough to keep pace with high end players. All MMOs do this -- but WoW handles it significantly better than any other major MMO.

You can just hang outside in the sun all day tossing a ball around. Or you can sit at your computer and do something that matters...