Both anime depict life after death, communities of the undead and some kind of a revolt against the God. The main characters are also similar in a way: they help the undead people to actually "die" and cease their afterlife existance. There are also dramas, high schoolers fighting against the system, lots of fanservice and comedyshit in both anime.
Perhaps it is the fantasy style of both series sets forth the premise of Angel Beats and Kamisama no Inai Nichiyoubi. In both worlds, the dead no longer dies and thus death is non existent. Additionally, there exists a protagonist who gets caught up in such a world and seemingly encounters another strange individual. There is the themes of supernatural and bits of action involved. Additionally, the atmosphere of both shows gives off similar vibes.
The series give a similar feeling when you watch it and the whole buildup and character development is well done.
Both have to do a lot with the after life and towards the end they both end up sharing an extremely similar premises. Both really try to tug at your heartstrings though Angel Beats does a much better job then Sunday Without God. They both are enjoyable pretty much from start to finish although Sunday Without God leaves much to be desired in its ending
In terms of the mild tone, use of simple characters and a few fairly intense moments of action and others of seriously emotional drama, Angel Beats and Kamisama no Inai Nichiyoubi are fairly similar. There are elements of supernatural concepts that specifically revolve around a form of afterlife, ambiguity surrounding the existence of a god and, unfortunately, similar pacing issues.
Both include the concept of living in a world where no one dies and the way they deal with emotional and drama scenes may share same qualities as the story builds-up. Main protagonist is also female. The feel of music and animation may remind the similarity of the message of its anime.
The same exact plot of having people living in some kind of "virtual world", while the main crew keeps rushing toward knowing the truth about it. Full of philosofy and drama
[May contain some spoilers]
~ take place at a world where is no life or death
~ main character wants to bring peace to the dead
~ have an ending you won't predict
Both series' are in a mystery, drama with a plot twisting ending, that WILL leave a question behind after finishing the whole series.
Angel Beats' main male protagonist traits are can be seen in Kami-Nichi's main female protagonist.(the particular one is "helping others no matter what") and both animes have a impressive art and OST.
Both series' story revolves around God, which in both anime, depicts God as the one who abandoned them or given them a unfortunate life.
Same theme of death and afterlife. Both have a similar theme of finding peace in order to move on. Both have eccentric characters and a changing story line that leads to the main climax