With 2+ weeks past since Eureka's release, here are my current thoughts on it as a lowly, level 6 character:
Pros/Things I like
+You can play as any level 70 job and you can gear up any level 70 job while playing as any level 70 job
+I like the EXP fairies. It's kind of fun looking for them, like a scavanger hunt.
+The NMs were okay, if a little bit basic. I've done a few high level NMs when I can get on the NM Train, including Pazuzu. I would say his difficulty is about the same as an current-expansion A Rank Hunt.
+It is making more people more supportive in the community
+It promotes some exploration
Cons/Things I don't like
-The grind can be monotonous. Vidu said it best in their posts, whether you're in the NM train or chaining normal mobs, there is little to no difference in what you are doing from level 1 to level 20. How the battle system is designed to be rotation-based doesn't really help this either...
-The NM Train is life, brah. I know the NM Trains are a player-made thing but Eureka's design really facilitates it. A lot of instances are focues solely on the NM train, especially higher level NMs, which is discouraging to lower level players, who have to either stand to the side and watch higher leverl characters do most of the work, try to help by pecking away at the NM for 20 damage while praying they don't get oneshot, or be a Phoenix Down as a Healer, RDM, or SMN (sorry if you wanted to DPS or Tank).
-The difficulty is front-loaded. I mean, it's to be expected that when you are at the current level cap, you're more powerful then when you started out and have a few more perks but almost all of the challenge in Eureka comes from level grinding. This can be discouraging to newer-or-lower level players, especially if most people in the instance are already at higher-or-max levels. I can't really talk for those who are higher-or-max level but some I've talked to and some I've seen on forums don't feel like the content is challenging/engaging enough.
-Visibility of monsters and partymembers needs to be fixed. They need to optimize the instance better so targeted monsters and partymembers don't dissapear during big, zurg rush fights.
Suggestions
=You could add daily and weekly objectives. Like hunts or quests. These could act as a buffer for some people's feeling of ennui during the NM train or chaining normal mobs. These don't have to just/only reward you with EXP, items, and/or gil either, they could give other things like temporary Eureka-only stat buffs or a buff to your EXP or maybe temporary access to special areas or something.
=You could take more inspiration from GATEs, having some less combative FATEs that revolve around platforming, traveling, stealth, and/or puzzles.
=You could experiment with 1.x's Hamlet Defense idea. Prepare a location for battle as crafters and gatherers and defend it as fighting jobs.
=You could add more interesting/difficult ways to spawm more rewarding NMs.
=IF the NM train is going to be the norm for the rest of Eureka, you guys need to figure out what to do with lower-level players. Maybe make cheering an actual mechanic? Like, have a special Eureka-only Cheer action you can do to other players and they will get a buff that will help them slightly? You could have differnt cheers for different effects. Just a thought...
=A Party Level Sync so Higher Level players could help lower level ones would be nice.
=Allow players the ability to turn in multiple Lock Boxes and Crystal at once. :B
My overall thoughts
Sorry guys but I don't like Eureka, which kind of sucks because it's the newest content we've gotten and the relics are locked behind it, plus it's the only way to gear up multiple jobs outside of a weekly-lock out.
I found it really repetitive.
While I personally don't mind NM trains and perfered fighting NMs over normal mobs, I really don't like how the train has become the centeral focus of the instance and that it kind of promote lower level characters to be inactive during fights out of fear of being oneshot.