Once, I began to wonder, what affects player's interest to the game. Yes, we probably know some factors, like quality, support and variety of contents and actions... That is pretty developed topic. But then - the next question, that looked more important to me - interest isn't some momentarily effect, then - how long will it last and how strong will it be?
I really don't want to start "creating laws" of mmo gaming or entering into game marketing topics, neither can I go on a complicated discussion about game's concepts. But i'd like to tell you about one intersting example, with which i was accustomed very well - to make my picture of the situation more clear and to give you some food for thinking.
This example is the game called RF online - korean mmorpg released by CCR inc. It is pretty old now - about six years passed since its release. Some of you probably played or even still playing it - Codemasters has been running european servers for some time ago. But it is still supported by other providers in different countries.

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Unique yet flawed.
Rf online, despite being point-and-click rpg game in its core, has some unique features. First of all, players are divided into three races/factions without any means for interacting or helping other factions, and just this division forces them to engage into the conflict. In fact, this conflict is the main and probably alone point of the game. Second, this races is special enough to have their original style, history and battle style. The balance is so thin, that you can go disputing for a long time about the most powerful race.
For the sake of this conflict the massive battles between the population of the entire server is held - and a possibilities for using various tactics and strategies are opened into the simplest mmorpg system, which don't even have class builds or equipment variety whatsoever.
That is, game with a concept of RvR and battles between large groups of players.

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[if you don't have a guild or a super weapon, you are still can help your race and participate in the war for resources]
Personally I was attracted to Rf online mostly because of the futuristic setting which is rarely carried out and the way like it was made into Rf online. Feeling of involvement into server's life by just participating into daily global wars was very nice, too.
So, you can ask, if this game is so cool and original, why isn't it popular enough for you to hear something about it? Well, I can't say that this game is entirely bad - as i mentioned, it features pretty unique systems and design, which is yet to be repeated anywhere else.
But, it's not that good either - with horrible graphic engine, massive bugs and just plain underworked moments, which are continuing for years with no fixes. Like, when the game crashes every time you enter some particular location.

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[you can suddenly find yourself on the roof, just because there is always some glitches in geographical data]
Long story short, It is made just like the game "you still playing" is made. You won't drop it next day just from boringness, you won't get addicted to it, spending months to achieve anything in it. Its just that, you will play it, if you still like something in it.
I played it for more than two years now totally - making a pretty stable community, watching this game's falls and rises.... and I finally started to see the parts of whole picture.
It is natural enough to think that all of its issues, like bugs and overlooked moments will be corrected over time, especially in the case of korean development style - you know, "Release first, make corrections and add contents second". But RF online is really the extreme version of this style.