Avengers: Earth's Mightiest Heroes
I really loved this show, unfortunately the people at Marvel (particularly Jeph Loeb), didn't like where the creators (Christopher Yost and Joshua Fine) were taking the show. Particularly it was felt that the show was becoming to serialised with to much back story, making it harder for people to jump in. It was also decided that the show was drifting to far away from the main three Avengers (Iron Man, Captain America and Thor), and that too much focus was on "minor" characters (ie. all the other Avengers). So half way through the second season a bunch of new writers were brought on and the rest of the season was primarily episodic episodes about Cap, Iron Man and Thor. Interesting plots about Hank Pym, Sutor and Vision were completely dropped in favour of more screen time to the big three.
But that's not all. They delayed the second seasons airing for nearly 6 months so that they could have it show back to back with their new Ultimate Spider-man cartoon, so they could attract viewers to that. Expect that show is extremely disliked. So then EMH had to go on hiatus, the reason given was so they could get an accurate picture of Spider-man's ratings.
(Luckily for me, it continued to air here in Australia. If you've watched any of last 15 or so episodes online, it probably came from our broadcast)
Even after all that it was still cancelled. The decision to cancel it, in favour of a new series more in line with the Marvel Cinematic Universe, was made before the changes to Season 2 were even finalised.
Wow, that turned out to be a bit of a rant. It just really pisses me of that such a promising show got cut short, with a very sloppy and rushed conclusion. Although I guess 52 episodes is quite respectable when it comes to cartoons now a days. However if the new show is terrible I cannot be held responsible for my actions.