Here's kind of my lengthy first impression after two listens:
This album kind of reminds me of Dark Passion Play. It has an epic, a couple of different-sounding songs, and a bunch of songs that all have a pretty similar sound to each. I think Endless Forms Most Beautiful is somewhat better from a songwriting standpoint than Dark Passion Play, though.
In my two listens, I've basically felt the album had two parts: Shudder Before the Beautiful through Alpenglow, and the last two songs. I'll talk about each part successively.
The main sequence of songs, tracks 1-9, definitely include a lot of songs with a similar vibe to each other, much like tracks 2-9 on DPP do. Fortunately, the songwriting is pretty good. Shudder Before the Beautiful is an electric opener. I know some people have gotten tired of them opening their shows so often with Dark Chest of Wonders, but in fairness, that song was a dynamite opener. Shudder Before the Beautiful (which is, in fairness, a bit derivative) is right up there in energy. Hopefully they open with it on the tour. Weak Fantasy is another really energetic follow-up that should be great live. As the album goes on, it does get a little tiring due to some of the sameness, but I would argue that each song is good on its own. I'd say Our Decades in the Sun, My Walden and Edema Ruh are highlights. Alpenglow is a really good closer for the first part of the album, kind of like how Last Ride of the Day was a good semi-closer on Imaginaerum. The music is pretty epic and due to that and the lyrics of the song ("Time to rest now and to finish the show / And become the Music, one with alpenglow"), I rather hope that they close with it the same way they closed the Wacken concert with Last Ride. Honestly, as I listened to this album, a lot of what I thought about is how these songs would do live. I think a lot of them will be just fantastic in that setting, especially the first two and Alpenglow.
The last two songs take the album in a whole new direction. The Eyes of the Sharbat Gula is a pretty cool instrumental that is quite different from Nightwish. To me it is a bit reminiscent of Tuomas's solo album, though not in any overtly derivative way. Kind of cool to hear some of that songwriting style in Nightwish. The Greatest Show On Earth is a whole new monster. I'm not sure I can definitively say how much I like it after just two listens, but it's definitely the most ambitious thing that Tuomas has ever written. It's a definite journey that travels through a series of moods and ideas. The intro in particular is fantastic, but the whole thing is an enjoyable experience. I'm just unable to say at this point where it belongs among the other Nightwish epics, because those are among my very favorite songs by the band.
I appreciate the continued presence of the Nightwish-style rhythm section. I suppose Kai Hahto has worked before as Jukka's drum tech? In any case, he does a pretty good Jukka (maybe Jukka helped write the drum parts to an extent as well?), and seems to be a good drummer in his own right (he grooves very well in Weak Fantasy). He works very well alongside Marco to maintain Nightwish's quality rhythm parts. Emppu seems to have a bit more of a presence here than on the last few Nightwish albums as well—he's still mostly understated as is his style in this band, but he does get a cool solo in Shudder.
Vocally, Floor is pretty good. I feel like some people had expectations that were a little too high for what she was doing. Personally, I did not set my expectations with reference to Floor's collected works, but with reference to how Tuomas is writing vocal melodies at this time. He's spent two albums writing for Anette Olzon, and even on Once he was moving towards a less operatic, more straightforward style of vocal melody. Maybe on the next album he'll stretch out a bit more to Floor's capabilities, like he did a bit on The Greatest Show On Earth, but the way Tuomas wrote vocal melodies on this album is pretty much in line with how he has liked to do that since at least 2004. And I think the melodies are pretty good. Anyway, Floor nails it. My standard of how good the female vocals are on this album is that I feel they are an upgrade from Imaginaerum, and I say this for all that I like Anette. Floor is just in another league. She also showcases quite a lot of versatility, in my opinion.
What's most notable, perhaps (and as we saw hinted at times on Showtime, Storytime, like Ghost River), is that she is basically directly challenging Marco over the aggressive parts. Marco was able to push around Anette vocally, but he can't do that with Floor, because she pushes back hard. She's just as powerful as he is, which I think is pretty cool. The downside here is that, as gazinwales said, we get very little Marco singing on this album, which I think is a by-product of Tuomas needing to find a new way to use him that isn't just "give him the aggressive parts that Tarja/Anette can't do." I think we saw the beginnings of that with how well Marco was used for texture at times, singing basically behind Floor, like on the Weak Fantasy chorus. Still, it would have been nice to have at least one Marco-led song, maybe in the vein of The Islander or The Crow, the Owl and the Dove. Hopefully next album Tuomas will figure out a little better how to get more Marco in vocally.
Overall, I can't complain about this album. On 9 songs out of 11, this is the Nightwish album I expected. It doesn't wow me, but all of those songs are pretty good. They're like a (probably) better version of the main sequence of Dark Passion Play. Really maintains Tuomas's generally high quality of songwriting. The last two songs don't contain much that I've never heard before from Tuomas, but they still bring in some innovations. The Greatest Show On Earth flows really well and is at least good enough to consider Nightwish's first foray into the 20+ minute field to be a success.
To wrap up (tl;dr): Endless Forms Most Beautiful is not the burst of creativity that Imaginaerum was. That is a lovely, exciting and diverse album that rises and falls perfectly, never getting boring through its nearly 80 minute duration. EFMB isn't near that in terms of sheer creativity. However, it is a solid entry in the Nightwish discography. The songwriting is quite strong, even in its experiments like in the epic, and the performances are good. Overall, I'm pretty happy, if not overjoyed, with this album.