The most painful thing for me is that the tone of the show is serious, but many silly events occur and they are played completely straight. Almost every character has a ridiculous/exotic backstory, with no justification for why so many unusual people happen to be involved. If it were a silly-themed show this would be fine (see: To Love Ru), but the show's serious theme demands some kind of justification for the craziness which just isn't there.
Piled on top of this is a heavy helping of some of the worst puns I've ever heard.
The other major gripe I have with the show is that the writing seems lazy. It pulls lots of Deus Ex Machinas to steer the plot where it wants it to go and conveniently forgets about things when they go against the intended story direction. Some examples:
When in the middle of the series they need a reason for a normal human to be able to take on a squad of alien attack robots, a new character shows up out of the blue and gives the human magical power stickers, with no proceeding explanation of who she is or why she's helping or even WHY MAGIC EXISTS IN THE UNIVERSE (it didn't up to that point).
Another example of this is when they introduce a character with a teleport ability. They have a whole scene where she demonstrates how she can teleport objects up to 50 meters. Then, in the very next scene a bad guy throws a flashbang at her and she just cringes and takes it instead of using the power she just demonstrated to teleport it away, resulting in the bad-guys escaping. If she were caught off-guard and had remorse for her mistake afterwords that would be fine, but she never even notices that teleportation was an option in that situation.
The leader of the good-guys has a multipurpose transmogrifier/organizer/weapon thing which she makes a big deal of pointing out is a weapon, and then despite it being involved in multiple battles throughout the remainder of the series it's never actually used as a weapon; Chekhov's gun is never fired.
It's not entirely bad; the character arcs are decent if predictable and unlike many harems the protagonist manages to avoid being a doofus (although he is a bit oblivious). The references to other SciFi and fantasy shows are a nice touch (including Star Trek, Red Dwarf, I Dream of Jeanie, and Bewitched), and the theme song parodies can be fun.
I would advise my past self against starting this series, as the pain of viewing it was not worth the payoff of seeing the ending. If I was going to do it anyway though, I'd let my past self know that the first two episodes are prolog and very different from the rest of the show.
