I tried to respond to someone's comment on why Lucky Star is so popular, but after submitting the comment, it just disappeared. So now I'm posting it here full on out.
In my opinion Lucky Star is popular because of the following reasons:
1. Lucky Star works under serious moe, and it's cast of cute girls attract both male and female genders. The moe alone can attract, but if that were all, Lucky Star wouldn't be as near as popular as it is.
2. The series starts off with the girls in a classroom like setting that anime fans can relate to, since most of them have been to highschool themselves. The first few episodes consists of them talking about pointless things such as food, dentists, etc, but this creates a certain atmosphere in the series. These conversations are not so different from the conversations many people have with their friends on their free time. So after a few episodes of experiencing this from the point of view of a silent friend sitting with them, it creates a stronger feeling of friendship toward the characters, rather then simple characters you are watching on tv.
3. In continuation with point 2 the bond of friendship is reinforced by Konata. She is the spirit of an otaku amongst "normal" friends. Everything she hears goes through a filter in her head. This makes it so that a simple conversation will be related naturally to something anime in her mind, and often causes her to make comments based of her though process. This usually works well in gags, but makes the anime fan feel a sense of "I totally do that too" on occasion.
With this said both male and female anime fans feel a stronger connection to the characters in this anime/manga series because it works brilliantly in creating this psychological feel of friendship between the characters and the watchers/readers, and a feeling of being able to relate to Konata's otaku minded filter would make anime fans love the series many times more.
The point of Lucky Star isn't to tell a awesome story of hero's slaying dragons, and it isn't even trying to make an adorable story either. It works from the beginning to create a relationship between the viewers and the fictional characters, and that is why I rate Lucky Star as my #1 anime. When I watched it I felt this was their goal, and looking at its popularity, I feel they accomplished it brilliantly. This combination of cuteness, the creation of friendship, and the feeling of relation is why I easily see many otakus falling in love with these characters, (as a man would fall in love with his or her friend). Many Slice of Life anime and games have tried this method as well, but I am personally partial to Lucky Stars method.
4. Lucky Channel is just too funny for words.
Those are my reasons, debate them how you wish.

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