> - Is communauté actually used instead of community?
Yes. look here for an example you will see communauté at the top right
You can also add a "de" for :
"Présentez-vous à la communauté de Fractured !"
I personally prefer without "de" is more younger style but both are right and a french teacher will choose with "de"
> - "des combats d'action" - the plural sounds a bit strange, is that how you'd say it? I would have guessed "combat d'action" or keeping the English "action combat".
If you dont use the plural that mean they have only one combat in the entire game so no. You can keep the english sentence but you need to use "" like :
"Le premier MMORPG Sandbox en monde ouvert proposant des "action combat", des environements ..."
Its ok but a french teacher would kill us and this time I follow the teacher. 
You need to keep "Des" but you can also use some other word instead of action like
"des combats trépidants" or "des combats à vous en coupez le souffle" or "des combats exaltants"
> - "Un MMO dynamique" - Hmmm... You don't use taglines line "Le MMO Dynamique" in French?
Yes you can use "Le" instead of "Un" both are right but for publicity "Le" is probably better because it targets precisely while "Un" is a generic
> - CLÉ for KEY - same question as for "community"
you can write "Clef" instead of "Clé" (yes in french we have two different word for the exactly same english word) but "Clef" is so old..
see an example
> - "manière de jouer" foe gameplay, same question...
Yes its a mistake because as you can see the first time in AD2 texts I have keep "gameplay"
"Tout les jours un nouveau Gameplay"
