THIS POST HAS LOTS OF SPOILERS!!! Series like Kuuga are very closed: The monster is dead, Kuuga's belt is broken, end of show, role credits.
The monster is dead? I never saw Deguva die. I just saw him and Kuuga fighting in a bland battle that just moved on to the next scene without showing the outcome.
The same goes for Agito's finale. I saw two balls of light fly around, not actually seeing any characters fighting it out and winning.
With that said, there is more to an ending than whether or not the hero still has their powers. To me, an ending is when the main villain is fully defeated (on screen) and all plot holes are closed.
W, OOO, and Fourze do this. I don't mind the way DaiRanger and Blade did it because they defeated the main villain of the show. In Dai, Shadam was the main villain, he was defeated. In Blade, the King Undeads were the main villains and all of them were locked away. Yes, there was Joker, but they found a way to solve that problem and do it in an unexpected manner. In the first Garo season, Kiba is defeated, Messiah is defeated, Cerberus is defeated, and Kaoru is saved, the end.
However, it is things like DekaRanger where Agent Aburera was hardly a fighter, and as such, the final battle was lackluster, that I do not like. In 555, there was no final battle, it just stopped. In Kabuto, they completely forgot about the Worms, who were the main villains for the first 44 episodes of the series. Instead, the show turned towards a small faction that turned out to be evil and then the show ended with their defeat. What happened to the main threat? The ones they were fighting since before the show started!? Hibiki never had them finish off the real guy and girl. Or Decade, that just left off at episode 31 and then tried to end the series with several different movies and still couldn't pull it off.
Those are the things that annoy me. Let's take some anime endings for example. My biggest beef besides Evangelion is the ending to the stupid Utena movie. That was the dumbest piece of **** ending I have ever seen. Then the director in the commentary track tries to make it sound so sophisticated with what it symbolizes, but if you have to explain the ending, it is a **** ending no matter what. I hate when these people try to make it sound complex just to make it sound like it is more than garbage. Does that mean I understand every ending to every series? No, but when the director has to explain it because of how much nonsense it was, because of how random it was, it is utter ****.
The other thing I don't like is the reset button ending. Unless it is pulled off very well (which is rare) I hate how it just makes the entire series pointless. Ryuki did it. Mai-Hime did it. Wolf's Rain did it. Big O, Evangelion, RahXephon, etc, all do it. It has just become so cliche and the scapegoat for when people don't want to take a risk at doing something different.
Anyway, I don't mind open endings, I just want my stories to feel complete like we get with things such as Gankutsuou, Escaflowne, Noir, Yu-Gi-Oh!, KakuRanger, Gokaiger, and CarRanger. I don't want to feel like I sat through hours upon hours just to be given the finger because either the studio forgot their show was going to end or they made it so convoluted that they had no idea how to end it on a proper note. The ending to a show/book/movie, is one of the first things that should be figured out before the story is even allowed to go into production.