Note: I'm aware the game is early access. Doesn't change the validity of my points.
So I spent an hour or two in Grimm Eclipse yesterday, and i thought I'd share my thoughts. (I've made it as far as the 4th wave near the Giant Gear in the Second Act, and tried out each character)

What I liked:

- Visually appealing
Its very faithful to the aesthetic of the show

- Combat
Its fun, but has issues (see below)

- The sound design (mostly)
Sounded beautiful, but had some odd issues.

What I didn't like:

-The price
I'm sorry, but what we got is not worth the $15 USD price tag. (thats roughly $20 AUD. Thats enough for 2 decent finished indie games on the Xbone)
And because I've bought it early access, that means I won't have to buy the final release, correct? (I don't do Early Access games, so this is beyond me)

- The leveling system.
Not so much the system itself, but rather the fact that the level system isn't a permanent one. Its purely only for your current match, which is a poor move for a Co-Op game that has no Competitive mode (since the whole point of that style of leveling up is specifically for competitive matchmaking, hence why its so popular in MOBA-style games)
However, since I haven't finished the campaign thats currently available, I don't know if only then will it save the characters progress (which is still a poor design choice)

- Lack of Lore
So far, in the first level, only Ruby actually names one of the enemies you fight (and its kind of the current Iconic Grimm atm, a Beowolf), whereas everyone else just says a line of dialogue.
In a game like this, naming enemies is very important, since it helps encourage teamwork.
e.g. How can I communicate to my team to stop the two legged reptile like Grimm attacking the Control Center? They don't have a name revealed in show or in the game (Hell, it was only through using a wiki that I found out they are named 'Creeps', and that was from the RWBY V2 commentary), so how are non-RWBY fans going to learn about the enemy types?
Thats poor game design, since the effort isn't being taken to teach the player necessary info.

- Lack of Unlockables
So far, the only 'unlockables' are 'Mastery' abilities in the Leveling system, which require a minimum of 4 or so levels before you can use them (thats basically one full level of gameplay, assuming your playing solo).
Given the nature of the show, and the nature of the fandom, there needs to be some form of useful or interesting unlockables, like alternate outfits, permanent abilities, extra Lore info, etc.
Let me have something to show for the time I might dedicate towards playing.

- The focus on online Co-Op only.
If it wasn't for the fact that I watched a friend's stream, I wouldn't have even been aware of the fact that it was possible to play GE in a Solo way.
However, this is also more of a personal gripe, since Australian internet sucks, and I hate having games that don't need an online connection, requiring an online connection.

- The lack of rewards
Similar in nature to the lack of unlockables, there is zero reason to full explore the levels, since all Crates and Artifacts do is give you more 'Only for this game' experience.

Issues that need to be fixed/things that need to be changed or added:
- High amount of weirdly placed invisible walls.
Especially since most of these oddly paced invisible walls easily could've been fixed by moving the wall back a few steps, where it would've been perfectly nestled in an actual wall.

- Aforementioned invisible walls actively discouraging exploration.
Early misuse of Invisible walls will easily discourage most players from exploring. And considering how Artifacts and Crates give you bonus experience only, why bother looking for them?

- Reward the players
So far, I'm not really inclined to play GE unless asked to by friends, purely because there is nothing here that makes me want to play beyond completing the levels once.
There's no actual progression, nothing to work towards unlocking, no customization, etc

- Make the characters feel different when fighting as them
So far, beyond the special moves, most of team RWBY fights in a very similar manner, with only a few small differences beyond their special moves.
They do already feel slightly different, but they need to feel noticeably different, not just a different variant of the same move sets.
e.g. Make Yang a bit slower than Ruby and Blake, but make her attacks stronger, or make her normal attacks chargeable (a la the Gilgamesh weapons from DMC4).

-Move the Health UI around and make it more noticeable.
Having the health meter being in the bottom left corner (and I mean the very bottom left corner), and having it so small, makes it very easy to ignore.
The amount of times I was put into the danger state because I was too focused on the combat and couldn't see the minuscule health bar was ridiculous.

- Change the Dodge button to the Left Trigger on the controller
Left Bumper isn't a good spot for that vital button. It doesn't feel natural, and from what I could tell, the Left Trigger isn't even being used anyway.

- Character select
Either make the names a small drop down box (a la Left 4 Dead), or give the characters a proper page explaining what their strengths, weaknesses and abilities do.
Because the current Character select page is a huge waste of space.

Verdict
5/10
Its got potential, but they need to change the direction away from this MOBA/L4D style of progression. It really doesn't work for what their aiming for, and honestly doesn't make sense to use for RWBY.