"World of Warcraft is a computer game known worldwide, which counts over 10 million people in its fan base."
["Well-known" is redundant here, as the game is played worldwide. As for "which counts" we need to have something to round this sentence off, using "among". I suggest "fan base", to avoid using the word "player", which crops up a lot here. It's good to vary things.]
"It made its first appearance on the marketplace eight years ago, and from then on its aficionados increased more and more until last year when its creators -- the Blizzard Corporation -- launched a new expansion. It was considered [by whom?] the worst revision World of Warcraft had ever known."
[I suggest "aficionado" to avoid another use of "player".
So has the number of players stopped increasing? Is the number decreasing? It might be beneficial to say why it was the "worst" expansion -- were there technical bugs; was the game more boring than hitherto...?
I changed "expansion" to "revision" just to avoid using the same word twice.]
"At any rate, since it is an online game, the community behaviour [interaction?] is as relevant as the originality of the game itself to keep players coming back for more: you never know what kind of person you may find in it."
[I suggest "coming back for more" instead of "playing" to avoid using variations of play, playing, player.]
"You will most likely be interacting with [other] adults because, unlike almost any other game [of this type?], it has to be paid for by monthly subscription."
[I'm not totally sure what you mean in the first part of the sentence. Is this game played by adults only, as there is a monthly fee? The way it's written, it makes it sound like children also play the game, but there are fortunately opportunities to play with adults as it's fee-paying?
"To pay" is irregular -- "paid" in the past tense.]
"While it is nothing more than the typical MMORPG in which you create your character, complete the required quests and fight other players, it nonetheless has something undefinable which keeps you hooked."
[You start with "However", and your second clause starts with "but" -- these are similar in meaning. I think you mean that despite it being the usual MMORPG (whatever that is!) yet it has this power, etc.]
"It is very worthwhile to play, but make sure you like it before you pay for it."
[Not sure what you mean to say with "After all" here. This expression is used to emphasise an argument -- "After all, I paid for the piper, so I should choose the tune." If you just want a general summing up phrase, then you could say "To sum up", or "Finally". Something is worthwhile doing, or it can be worth your while to do it.]