Ok, lets see if I can shed some light on here. Since about only 3-4 people in this entire thread (skipped most of 2nd and 3rd page because of utter fail) have any idea what an MMO is, I am going to try to cover most of the points. Ive been playing MMOs since the beginning, though I haven't hit them all. Nexus, EQ, AC, plethora here, WoW. Wow is the best I have ever seen. It isn't the best in every category, but it does just about everything and most thing it hits on, it does very well.
Ok, first off. Everyone and their mother is complaining about the cost. Are you kidding? You get servers that are almost always on and fast that can handle thousands on the server, and hundreds in the immediate area. No PC could host a game and do this. For what you pay, you get massive amounts of time. 700~ hours a month for only 15$. Even if you play nearly max free time (8hr/day), you are only paying $0.06 an hour. And $0.50/hr if you play 1hr a day. That is by FAR the cheapest entertainment you can buy. It also costs the same to buy WoW and the expac and a year of subscrib as it would to buy Xbox Live gold, and 4 games in a year. So really, for the amount you get, you are getting a pretty good deal. But if you dont understand why you have to pay to play an MMO, you don't belong there anyways. Which is fine, because we don't need more FPS type gamers in MMOs. It already shows.
People also want a game where they have an effect on the world. Ok, lets be realistic here. You can have an effect on the people, not on the world. When you have 5-6000 people on a server, if everyone had an effect, it'd be utter chaos. If the server only had 500-1000 people on it, it would either be empty or too small to keep you interested. There was a game that tried this. Shadowbane. I never played it, but from what I heard, it was a very good game that never got popular.
People who don't like levels. There's been games like this. UO and AC. In AC, you had levels, but it was more of an estimate of power. In that game you spent ALL of your XP to increase your stats and skills in whatever way you wanted. No auto-leveling of skills or stats each level. Therefore, a L40 could easily destroy a L60 if the 60 misspent their points. This also covers games with no classes and more personal choice on how to customize characters. This was also a great asset of UO and AC. The problem with this is that it is great for people who want to be different. But if you wanted to be competitive, it doesn't take long to realize that you need to min/max. So while in AC you could do anything you wanted, in the end you really only had about 4-5 choices to be really competitive. Forced classes that are all viable make being different more of a possibility for everyone.
You don't have to play an MMO all the time. I don't, yet I play WoW. I have fun just playing 1-2 hrs at a time. Going slowly and learning a new class. Many people do this. Many people play Halo, WoW, and CoD4 in the same day. $15/mo isn't that much so you feel like you HAVE to play it constantly.
Ending to the story? No. That's silly. MMOs give you a WORLD. Not just a game. In real life, when the Iraq war ends (eventually), should the entire world come to a screeching halt? Of course not. Just the 911/Iraq war STORYLINE came to an end (not that great of analogy, but its there). The world kept on. And MMOs do too. The devs roll out more storyline and keep it going. Its not like a single player game where you wait for the next sequel to come out.
Fantasy. D&D made MMOs and such popular, so its natural that fantasy would be the dominant universe for this game. Cold war, sci-fi and ancient armies made RTSs popular. Which is why you see so many of them and so few fantasy RTSs (warcraft was early, but most are not fantasy based). Also, there are sci-fi and modern MMOs. They just aren't all as good as the few really great MMOs which happen to be fantasy.
Real time combat. Its been tried a few times, but lag is mostly the issue. When you only have to keep track of 5-20 people and can switch servers to find one or players close by with low lag, then lag isn't a big deal for twitch-response. But when you are locked into a server with thousands of people and NPCs to keep track of, then lag is more of an issue. Yet, the most popular MMOs is quite twitchy. If you aren't constantly moving and turning in PvP, you are dead very fast.
I probably missed a bunch I wanted to cover, but I'm getting tired of writing. In the end, MMOs aren't for everyone. If you don't like them, that's fine. I'm not the biggest FPS fan, but they are fun once in a while. Every genre will have its pluses and minuses, and every genre will have fans.