I'm going to do my best to keep this review as objective as possible. Of course, I might have to get a little subjective (though I will try to keep as many personal opinons out as possible) in the Overall section to explain just why I rated it so low.
All in all, I wanted to like Digimon Zero Two, and it was a fun anime to watch. However, and I say this all the time, there are just too many plot holes as a result of bad writing. I'm of the opinion that one could fly a 747...no...perhaps the Millenium Falcon, through the rift of plot holes.
Story
Zero Two picks up three years after Adventure left off, but includes most of the original cast as side characters who act more as mentors for the new kids (except in the World Tour arc, where they played an active roll). This is because the digimon can't evolve normally with the appearance of the Dark Towers in the Digital World, which requires the digivice to undergo a transformation in order to include the new evolution (along with a power phrase to activate it): Armor Evolution.
Something is wrong with the Digital World, and this time, it's in the form of not a digimon, but another child; and not just any child, but another chosen child/digidestined.
The anime starts by introducing us to those characters, complete with name bars on the screen, as we follow Takeru (TK), who has just moved to a new apartment, as he meets the soon-to-be new chosen children and reunites with Hikari (Kari).
From there, Takeru and Hikari have to deal with this new wave of kids, fighting an opponent who is human, and then even getting a glimpse of a mysterious location referred to as Dagomon's Ocean (or The Dark Ocean).
Of course, that's not the end, as an old enemy returns to reveal that it had been responsible for most of the havoc wrought on the worlds.
Characters
Zero Two has probably the worst character development with the exception of the new chosen child/digidestined. In fact, the only reason why I gave it a higher score than "1" is because of the one character who actually shows the most development, Ken.
Ken Ichijouji is a new chosen child introduced in this season...who oddly enough, has the crest of kindness even though he didn't appear in the first season. Practically all of Zero Two is Ken's story, and his transformation of character throughout the season. He is also the most developed character, and seriously the biggest reason to watch the series.
Takeru Takaishi (TK) is a returning character from Adventure, and one of the two from the original cast who plays an active roll in the adventure. With the exception of a couple times he grows angry at the darkness due to the scar of what happened in the fight between Angemon and Devimon, he appears to be the overall "nice guy" of the series, though until Ken joins them, he's painted as (a failure of) a lone wolf.
Hikari Yagami (Kari) is the other returning character from the original Adventure, and also has next to no character growth or development with the exception of a revelation which occurs within the span of a couple seconds (no joke).
Daisuke Motomiya (Davis) is the new "goggle boy" of the kids, though he literally has no character development at all. Any changes to his character (also referred to as "sage-Daisuke") is done only if the plot needs it. He has inherited the traits of courage and friendship, and gained a partner in V-mon (Veemon).
Miyako Inoue (Yolei) is another of the new kids, and the eldest. She is the hardest to describe, as she also goes through almost no character development and is also just there when the plot needs her. She's hyper, reluctant at first (but quickly amends that). She inherits the traits of purity/sincerity and love, and gets Hawkmon as her partner.
Iori Hida (Cody) is the final chosen child of the new cast, and is the youngest (at nine). He has a lot of potential to grow as a character, but only shows slight growth during the DNA evolution saga along with his slow coming acceptance of Ken. He inherits the traits of faith/reliability and knowledge, and becomes partners with Armadimon (Armadillomon).
Sound and Animation
I'm going to briefly touch upon the sound here.
I prefer the Japanese version, but the dub actually did a fairly good job...except that it did include some light humor to lift some of the dark themes Zero Two attempted to bring into the series. It also changed some dialogue completely in episode 13. However, the voice actors did a fairly good job, and the English insert songs weren't bad. The intro/exit and insert/evolution songs were also excellent as usual.
The Jogress/DNA evolution, though repetitive like every evolution scene, was awesome to watch, and the CG used on some of it was very well done. However, not all those were done in CG, so I felt I should dock a point for that inconsistency.
Overall
The story sounds great from how I worded it earlier, but it's really where the series crashes and burns, because there are more questions raised than they are answered as things progress. The first arc, which spans episodes 1-23 (out of a 50 episode series) goes on for much longer than it needs to be, and from episodes 24+ includes the writers trying to cram in new plot points which just falls flat on their faces because there isn't enough time to truly make a good story out of it.
There is little to no character development for anyone except Ken, and there are dozens of points brought up during the later arcs which needed much more time and episodes to be addressed.
One point was even brought up in episode 13, where Hikari is transported to Dagomon's Ocean. This episode actually features Hikari and Takeru, of whom we should have seen in the spotlight a lot more during the adventure. If the first season could properly give a cast of eight kids and their digimon (making the total 16 plus the bad guys) enough to learn about them, then a cast of five plus their digimon (later on, six) should have been easy-peasy. Now, I'll just say that this episode was actually written by different people who did the rest of the series, and it's unfortunate that these people couldn't have written the entire season.
To me, Zero Two felt like a trial run for Tamers, which succeeded where Zero Two failed in the theme: something which is a lot darker than one would expect/has dark elements to it.
Dagomon's Ocean and the origin of the Dark Towers was something which should have had a lot more explanation, but failed miserably. We also start to wonder where some of the antagonists have been for the first 23 episodes of the series.
It also lacks the survival element, as in Adventure, the kids couldn't go back and forth between worlds as they pleased, and were forced to deal with what came their way. I feel like the series should have either started out like Adventure and trapping the new kids in this strange situation, or done that towards the latter half when things spiral into out of control craziness which makes little sense because all of the "arcs" are too short to really provide enough substance.
Unlike in Adventure, the parents don't have much of a role in Zero Two. There's also the issue with the Dark Seed/Spores, and what led up to that. I had to do research to get information on what all that was about, and came up with a game for the Japanese WonderSwan called "Tag Tamers", which explains the backstory to all of that.
Online Digi-dexes also gave a better glimpse at what the Demon Corp (a group of enemies who show up in episodes 43-45 and really should have been more of a focus) was trying to do, as again, research was required to fill in the blanks which the series failed to explain.
There were at least half a dozen plot points brought up (the Demon Corp included), and only a couple of them actually came to an end. Not a conclusion, as they were wrapped up poorly (specifically, BlackWarGreymon), but ended. That, of course, left the other plot holes wide open even as the worst epilogue possible was shown.
I have a lot of issues with the last couple episodes; I'm not going to go over them because I try to avoid spoilers in these reviews, but let's just say that, though touching the first time around, once it hits the epilogue, all of the questions start coming. Sage-Daisuke also pops up here, when it really should have been Ken or Takeru who gave the lines he did (heck, even Hikari could have done it).
The final boss is actually quite a disappointment, and sort of makes you scratch your head a little at first and go "are you kidding me?". So, besides the entire season in need of a rewrite, the last couple episodes were the worst.
The story had a lot of potential, but because none of that potential was actually utilized, I gave it a low rating. The biggest reason to watch this show is for Ken's story, which is pretty much what Zero Two is about. There are also glimpses of possible story arcs, but unless it's picked up in the 2015 season which was announced, that's left up to the imagination of fanfiction writers to correct.
It's worth watching, but the best thing to do is sit back and enjoy the ride; just try not to think too hard when you watch it, and you'll enjoy it. Re-watching it, or viewing with a keen eye, will bring up many questions which you won't find answered in the show itself.