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One of the things I do while following the zerg in WvW (other than watch TV shows) is to talk on mumble with my friends. One of the recent discussion point was 'why is GW2 so zerg focused?'
It is actually a game vision that made it so. The vision of GW2 is all about 'groups without the barrier to get players into groups'. It is so focused on getting people 'in a group' that just being in the same area = group in terms of gameplay; esp DE / WvW. Being in an area with a lot of players = more XP / Loot in PVE and higher chance to win in a PVP (sometimes referred to as 'Death Ball' or just 'Zerg' in WvW). Thus this encourages 'zerg' at a gameplay level because of the overall vision of the game and the game designs / game systems created to suit that vision.
Offensive is passive while defensive is reactive / active. The strength of your 'group' increases in a linear fashion as you add more players; hence it is passive. But all defensive systems are active; dodge, shield, heal, react to offensive push in WvW etc. Once again, the game pushes the player-base to be all about offensive and 'find the biggest zerg'. It's system is based upon trying to create the biggest 2 blobs of players and smash them into each other.
Whether you personally find this enjoyable or not is completely subjective, I find it amusing in small bursts.
Informing people about your thoughts and impressions is not a review, it's a blog.