My big thing with anime endings is that they should reflect the general feel and tone of the anime as a whole.
So if, for example, your show is all about kicking reason to the curb and overcoming seemingly impossible obstacles, then your ending should be about kicking reason to the curb and overcoming seemingly impossible obstacles...
If, on the other hand, your show is about a group of friends being forced into a Mortal Kombat-esque competition against each other and it results in a rather apocalyptic scenario playing out then maybe your ending shouldn't be all sunshine and lollipops.
