well i can not realy chose one (despite that i voted :P )
but i would talk a bit of each
city building) is realy good that if combined with other factors might turn this a good game however is not the city build that alone makes me be in the game
trade) i love trade so without trade this would not even be a game i would like to play once again the fact this game have is quite great but still don't meen i will play just for that once there are a lot games with trade
diplomacy and military) it is realy inovator the fact there are this 2 seperate ways of hostilizate with other players and in military case there are a lot to talk abou once it is quite realistic and of course there is normal that can be camping troos and that must be something to call them back is just no realistic that they can only be 15 days or that diplos can't or that they consume the gold at the presise time being in the city or miles away however this is a game and can not be totaly realistic because if it was my gold would be worth something out here :) this is defenetly an extreamly positive aspect of the game and combined with the last 2 WOULD make me already play this game
however we don't stop here (thats why this is not just a good game)
quests) well quests is a bad point is not negative once is each people desision to make them or not but is not positive and we can all see that once we are trying to buff tavern are also other players taking it down to get some more T2 buildings quests scores are on average prety low and is not as much hard to get in top20 as any other (ex:pop)
unless of course we are talking about misteries in wich case i thing is a realy positive point (i am frekye by trade and misteryes)
comunity/social) well i must tell that when i started playing i thought lets see if this game is realy good... well i am not much violent however i was used to games beeing violent and having to protect myself even in the first days... thats why i picked orcs (i thought that people would think twice before attacking a orc) and they did because untill now i was never attacked/stealed
i found it to be strange there was nothing in toturial about troops so i has forced to go forum look for the info and that was when i found the maravilous post by mananaaaa and i got it i went to GC and i recived 50k gold when i had only 20 or sothing also lots of basics... etc
without this HUGE BIG positive point i would have left the game because i were short in time and could not spend so much time in pc so this was the big point that made me realize there are other good points so i picked this one