These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Birdy the Mighty
Better on DVD: Episode 7 of Season 2's quality is increased immensely compared to the broadcast version.
Ear Worm: "Kiseki", the opening to Decode's second season.
Also "Future Shock", the ending to the OVAs
Evil Is Sexy: Revi isn't by any means ugly and does tend to wear does pretty sexy outfits.
Shyamalan's death at the hands of Sayaka after the Ryunka complete possesses her is a lot more unintentionally hilarious to watch when you remember that their seiyuu, Kenichi Suzumura and Maaya Sakamoto, are now married.
In the OVA series, Birdy is followed by someone who turns out to be a talent scout, thinking she could be an Idol Singer. In Decode, Birdy has a disguise as one.
There's an episode in which a little girl who defends a boy from bullies at a refugee camp Tsutomu and his friends visit sparks a debate among the boys about whether it's humiliating to have to be saved by a girl. In Heat Guy J, an earlier work by Kazuki Akane, two of the important men have the following exchange:
You have to be saved by a woman? That's so pathetic.
It's not every day you get saved by an angel.
Yutaka Izubuchi served as the creature designer for the OVA adaptation. He would also serve as the creative producer for Decode.
Moral Event Horizon: Nataru crossed this after killing one of the escaped aliens after she decided to leave the rest of the survivors in protest over their actions.
To be fair, she left less in protest and more out of fear that Nataru would find her if she stayed with the group.
Tsutomu's death. Especially in Decode, where he gets blown in half. In the OVA, he's electrocuted.
He's also electrocuted in the original manga. In the remake, he gets a hole blown into his torso.
Geega's transformation in the OVA.
In both series, Bacillus. He bonds himself to several dogs at once in the OVA. In Decode, his hosts visibly fall apart while fighting Birdy. His death in both the OVA (getting set on fire) and Decode (getting blown up) could also count.
In the OVA, the aforementioned mad scientist's experiments.
The escape of several aliens in the opening to the second season of Decode involves them killing the guards in goresome fashion. The subsequent deaths of several of said aliens in also horrific.
Violin's death in both series. While she suffered a severe head blow in the OVA, in Decode, her head gets crushed in.
Toy Ship: Marina and Takumi from Decode 02 episode 3