Of all the MMO games like Guild Wars 2 I've tried The Secret World definitely takes the cake as the most strange and unique. Just like GW2 the game is a paid experience but once you've purchased the game there isn't any on going subscription fee (although there was on launch so anything you read about that is outdated).
I'm a hug conspiracy buff and with factions like the Illuminati, Dragon and Templar I just knew that I was in for a real treat with the setting of The Secret World. It's a setting that focuses on these individual factions, myth, conspiracies and even some darker horrors, it's literally my ideal MMO setting and it's a great change from the never ending fantasy and medieval style MMORPGs which I'm definitely sick to death of.
I also really like how The Secret World approaches class options with the ability to switch instantly between classes, without these usual limits it's easy to create something with unique weapons, spells and abilities that wouldn't be possible in other MMOs.
Advancement through the game also provides several paths so you won't feel like you have to quest or grind which are generally the only two options in early-mid levels of similar games. Instead The Secret World offers you more choice with a faction specific quest line to follow, individual dungeon quests, faction side quests or a handful of long running side quests. These quests aren't just a standard affair either with a more puzzle and investigative process that makes it somewhat like a detective game in parts.
As a non subscription alternative to Guild Wars you can't miss out on the unique gameplay and setting that The Secret World has to offer.