Vann and I have had a talk like this before on these two games. This is mainly going to be about TERA since I haven't delved enough into FFXIV to give a proper informative response but maybe if Vanndril pops in, he can explain it.
I play TERA and almost solely warrior, so keep in mind the inevitable bias. The main reason I enjoy it being that I don't like auto-attack MMOs that feel all point and click-ish. It's a personal preference that has probably come from the years of spending time playing straight-to-meat-and-potatoes arcade games.
The main shining point of TERA is, of course, its real-time combat system. Of course that isn't to say that it isn't flawed in some aspects, but it's engaging enough and requires a good deal of approach and coordination with teams when taking down boss monsters. There is no guard/block/dodge-chance. It all revolves around taking advantage of skill-invincibility windows and actual guarding which will always block the attack (unless your gear is under-leveled). You can't get too comfortable though, being careless with those skills can potentially gridlock you and leave you vulnerable during cooldowns so you want to have an idea for a rotation of defensive skills. The game takes a good deal of skill in this regard and the intensity of boss fights can really test you and your team's endurance.
The main problem I have with this though is that if you're tanking the boss and pulling aggro, you're going to be right up in front of him. There's a set behaviour on the boss where if RNG decides it, the boss will displace and move to a different location. This particular behaviour renders its collision hitbox non-existent and, thus, it will be able to move THROUGH you. While doing this, it will rapidly deal damage to anything that is inside of it since damage is dealt every set amount of distance it covers (the bosses are large so I guess you could say every "step" it takes, it will deal damage). This behaviour is very sudden and there is no real charge-up animation. It's not a really well-designed behaviour for a boss and the fact that every boss can do it annoys me. It can catch people off guard and can really screw a team badly if it ends up moving through light classes like cloth armour users (priest, sorceror, mystics). Since cloth armour users generally have a low balance modifier and a low defense, they can easily get knocked down from this attack. Not only that, they are still vulnerable during the knock-down animation and their balance modifier is drastically lowered during knock-down so they can repeatedly get juggled for high damage.
So shining point + bad design aside... let's talk about the rest of the game.
When you first play the game, you get the usual initial-shock factor and I found that it lasted me a lot longer than other MMOs. The visuals are a real treat that add to it and the music is fairly ambient that help blend the experience together. However, after the shock wears off you quickly find yourself in another typical K-MMO quest-fest. You realize that you're not exactly exploring but that you're being guided to the next area via a quest. It follows the formula:
=> Reach x Level
=> Go back to Velika, Allementheia, etc. for a new story quest branch
=> Quest takes you to a town in a new region which piles quests
=> Finish every quest to unlock a new quest which requires you to speak to a new person at a camp further into the region
=> Arrive at camp and get a new pile of quests
=> Repeat until you reach the instances for that region
What doesn't help is that the quests are pretty much boiled down to "Kill x amount of enemies", "Gather x amount of resources", and "Obtain x amount of items by killing y and z enemies" with the occasional "right-click x to do y". It's all largely filler and it becomes a tedious chore. It feels like there could be depth there, but it lacks just that. This usually happens after you get a taste of your first intense boss monster around Lv18 followed by your first instance run around Lv20. The entire time you're killing monsters, you're constantly wondering when will the next boss/instance show up.
After a while of playing, you'll notice enemies get re-used as well. They just have a different adjective before their name and sometimes re-skinned into a different colour which apparently governs what level they are. Enemies you first fought at level 10 could suddenly be level 50 and because of that it feels jarring to just fight the same beefed up enemies that have the same attack animations you're already familiar with, the only difference being the damage, defense, and health parameters ramped up to scale into their appropriate level. This is also done for boss monsters.
The level-cap is probably like the half-way point for your character. Level 65 (it was recently raised from 60 in the expansion) is where the gear broadens. The game basically becomes a grind-fest for running instances to get good gear drops to run more difficult level-cap instances. And then there's the matter of enchanting it. From there, that's pretty much the entire game after that, not including PvP.
There are other systems like alliances dailies, and gear crafting, etc. but those I don't feel the need to make a mention of since they're not really big in the game.
Despite the game relying on its engaging yet over-appreciated combat system as the glue to hold the rest of the game together, I do like what EME has been doing. The game has never seen a day of Pay 2 Win (unless you count slight bonus XP gain but level cap will eventually come and gear governs the at that point). Most of the pay stuff is for cosmetics on making your character (see: lolis) look cool and even then, those can be bought with in-game gold from other players via the brokerage.
Lastly, the game's community is pretty active in socializing so thankfully if it's Friday or Saturday night, Global chat can get pretty NSFW in discussions and can help numb the grind (I'm surprised the mods never do anything -- actually are there even any mods that are supposed to moderate the chat?).
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Final Fantasy FFXIV. I can't say too much about it since I've only delved like 20 hours into it during my trial but from what I've played, I find it very much enjoyable.
This is coming from someone who dislikes auto-attack MMOs because "it isn't engaging enough to keep my interest". I found that for the amount of polish that went into FFXIV's interface, I was able to really get into it despite the lack of engaging combat.
Though one thing that bothers me is that your skill attack animations overlap your auto-attack animations. That and the map being absurdly difficult to use for navigation in a city.