All joking aside (as legitimate as it may be), I really enjoy TERA. And given my longstanding general dislike of Korean MMOs, that's pretty big for me.
TERA has a lot of neat things going for it. It has a very cool Guild versus Guild pvp system... a nice guild leveling system... a neat political system whereby people can be elected "senator" to rule over regions in the game... a very intricate crafting/item upgrade system with lots of little options for customization, etc. However without a doubt, it's all pretty much carried by the real-time action combat the game features. Everything pretty much balances on the shoulders of this core mechanic that sets the game apart from other MMOs... and it's a solid foundation.
Rather than locking onto an enemy and mashing your skill buttons, you have to aim your weapon. If your weapon contacts its target, you hit. There are no random number generators working behind the scenes to decide whether or not you win a fight. If you get hit, you get hit. If you dodge, you avoid damage. Your skill as a player overrides any skills your in-game character might possess. If you don't actually block an attack, your character won't magically do it for you.
Last night I was playing through an area, and ended up with a quest to kill a BAM (big ass monster). It was a group quest for five people, and the BAM was a party monster. I didn't realize it until I'd attacked. It took me nearly twenty-five minutes, but I was able to solo this monster a level above me and intended for a party of five adventurers, solely because of the combat mechanics. Learning its tells and attacks, I was able to dodge and block, circle around it, slowly whittle it down. It didn't autolock onto me with attacks I had no hope of avoiding. I beat a monster I wouldn't have been able to beat alone in any other MMO, just by playing smart. It was a lot of fun to go toe to toe with this BAM, and while I certainly wouldn't spend a half an hour soloing monsters all the time, that it was possible is a testament to the action combat.
This same action combat is what makes PVP in TERA so interesting. It's not about who has better gear, or who can cycle their DPS rotation the fastest, but positioning and strategy. And that's important because so much of the game is PVP-centric.
The questing, without the combat system, would be incredibly dull and standard for an MMO. Talk to person A, go kill a bunch of Enemy B, come back receive reward, etc. It's nothing special all on its own. But the action combat is so much fun, I find I don't even notice. Different enemies have different attack patterns, and so it's always a different fight. Additionally, your health regenerates incredibly slowly out of combat, putting an even further emphasis on playing smart, and avoiding damage where possible. It's not like other MMOs where you can facetank the damage, narrowly kill the monster, and then be at full health ten seconds after leaving combat and ready to go for the next enemy.
The combat is also the primary feature that helped TERA win out over Guild Wars 2 for me. Don't get me wrong, Guild Wars 2 is a really solid game. If TERA didn't exist, I'd totally be prepping to play that in a few months. However, having played GW2, I found that being a solid game just wasn't enough. While it did everything well, it didn't do anything that "wowed" me.
The Old Republic was a very standard MMO in most ways... but their cinematic questing set the game apart, and made the journey from L1-50 the most enjoyable I've ever had in an MMO. The interactive story was compelling enough that the tired old button-mashing MMO combat didn't matter. On the flip side, TERA goes back to the age-old boring questing format of reading page after page of quest text, which isn't at all interactive. But their combat system is compelling enough that you can overlook that.
Guild Wars 2 has neither. It doesn't have the interactive cinematic questing (the closest you get is the occasional, stiffly animated "talking heads" cutscene on a generic background), and it doesn't have a combat system that does anything truly different either. The features are well implemented, but they're features I've seen in so many games before. Nothing about the game really felt all that new and exciting.