I'm sure everything I could say has already been said, but I just feel I have to add my voice to the chorus. Eureka is awful.
I can get behind the idea of a nice big zone filled with dangerous enemies, where the idea is to form impromptu groups and go on adventures in killing mobs and getting loot. This kind of seems like what Eureka is intended to be, but in practice it is just miserable.
Firstly, the worst thing for me is that there is absolutely no variety at all. It is JUST trash mob grinding. There are no quests, no fates, no leves, no dailies, no hidden spawns or loot, no overarching or randomized objectives of any kind. You go in with your party, pick a group of mobs, and kill them over and over and over, for hours. Occasionally a NM pops and you all run over there and kill it because they're the only things that give decent XP, and then it's back to mob grinding. The mobs aren't challenging, or special in any way, they're genuinely just like dungeon trash. There's no special abilities or mechanics to be aware of, just the same AoE-dodging and tankbuster-defending we've all done thousands of times by now. There is a questline, but you need to level up to progress it, and the only way to level up is, you guessed it, mob grinding.
The elemental system doesn't improve the situation at all and the magia board on the whole seems half-baked. The strategy of it is obvious. You stack all your magicite in one element, and spin the board to match the weakness of whatever you're fighting, or the defense element if you are the tank. That's it, that's the entire system. There are no mobs which force you to consider an unusual magicite loadout, or mobs which force you to spin your board on the fly as you fight them. Maybe undiscovered ones at a higher level, but certainly nothing immediately available. As is, the entire thing just seems pointless, I don't even understand why this system exists, or what it adds. You could do away with it completely, and the gameplay of Eureka would not visibly change at all.
And thirdly is of course, the utterly, utterly absurd grind. Grinding is something that isn't uncommon in XIV, but I don't know if it's ever been done on this scale before. I'm not sure if this was done to intentionally resemble the grindy MMOs of the past, or if they genuinely think this is a fun and balanced approach to content reward schemes, but it is absolutely miserable.
Grinding mobs aggressively, you can expect to get maybe 20-50 protean crystals per hour, depending on your luck. However, you need THOUSANDS of crystals to get a fully upgraded set of gear, translating into literally HUNDREDS of HOURS of mob grinding. Just mob grinding, of course. And then this gear is not even as strong as the gear that can be acquired with tomestones! So not only is tomestone gear easier to get, it's also more fun to get, since there are actually many various ways to get tomestones that aren't just killing trash. What's more, because Eureka is synced to ilvl 300, this gear that you spend so many hours grinding out will not even help you INSIDE EUREKA! Even the tome-grinding of the Heavensward relics was preferable to this, because at least there are many ways to get tomestones, and besides that the grind wasn't nearly half as long.
You could potentially argue that this gear will be upgraded to be more competitive in 4.3, but that too will require even more grinding, so what is the purpose of having us suffer all this now just for mediocre gear? Even the cosmetic rewards from lockboxes are disappointing. Mostly you just get materia or worthless sparklers and prisms. The only glamours I've seen so far, are just crafter/gatherer leveling gear with the required jobs switched so battle classes can glamour it. That's pretty poor.
It is possible that the crystal drop rates increase dramatically at higher elemental levels. But that is just as big of a grind! After you've gotten past the first few levels, you're still looking at dozens of hours of mob grinding to even get as high as level 9 or 10, let alone 20. You're probably better off not grinding mobs at all and just waiting for NMs to pop, because for some reason NMs give literally 100x more experience than mobs. It would take maybe 1-2 hours of grinding to equal the experience of a single NM that can be downed in 5-10 minutes.
Lastly, a comment on the general state of content in the game. For some reason, the developers seem very obsessed with grinding trash mobs. Every time they "take a risk" by implementing a new form of content outside the normal dungeon/raid schema, that content ends up boiling down to mowing down easy monsters for hours. Diadem, PotD, Aquapolis/Canals. The context varies slightly, but ultimately all you are doing is grinding trash mobs over and over. It gets old FAST. I don't understand why this is. It's like the designers are afraid that anything more challenging than killing trash mobs is too hard for the players.
Is it any wonder that so many people consider the raid and trial bosses the only "real content" in the game? Besides providing a much nicer visual spectacle, they are also actually -engaging-. You're not just standing still and banging out your rotation on easy monsters over and over. There's mechanics and special effects you have to react to and work around. The game rewards you for playing well, and punishes you for failure. It's multiple times more interesting than just killing normal mobs.
When the event instances for Halloween and Valentine's day were released, they were very well received. People asked to see more of that kind of thing in the game. Why do you think that is? It's because these things are something actually DIFFERENT. It's not just grinding trash mobs! They're not hard at all, but the sheer novelty of the content made them unbelievably refreshing. Why can't we see this kind of thing in normal content as well? Why is it always just killing trash mobs?
At this point I don't expect to see significant revisions to Eureka's design, but I'm hoping at the very least that the grind will be toned down, because I definitely don't see this lasting long in it's current state.