Tenshi no 3P is not only just Ro-Kyu-Bu in a Jpop-rock band, they even featured the anime in one of the episodes!
Both shows you get enthusiastic lolis, and both of Kanae Itou's characters here are major-supporting characters who secretly loves the [boring] protagonist.
A teenage boy is tasked to help some little girls become better at something. In Ro-Kyu-Bu!, it's coaching basketball. In Tenshi no 3P!, it's playing rock music. Both anime are extremely cute and have amazing art. There's also plenty of fanservice in both, though Ro-Kyu-Bu's fanservice is a bit heavier.
A teenage boy is tasked to help some little girls become better at something. In Ro-Kyu-Bu!, it's coaching basketball. In Ryuuou no Oshigoto!, it's playing Shogi. Both anime are extremely cute and have amazing art. There's also plenty of fanservice in both, though Ro-Kyu-Bu's fanservice is a bit heavier.
Girls taking recreational activities in a school setting. Sparkling shining exciting fun enjoyable moment fun of fun girl fun life. Incite pedophilia, increase heart rate and make you go all Humbert Humbert. Apply to only men and lesbians.
Mood, tone, cast, genre, formula, target, how god classify this show are all overly similar. True, the degree of pedoness, funniness, ecchiness and any-ness may vary, but iPhone 1,2,3,4,5,6 or Nokia 1280 all can make a phone call.
Five girls in a sports club, led by a man. Simple. Personally, none of them is of my taste, but "Softenni" shows a cast and humor slightly better, while "Ro-Kyu-Bu", lacking any innovation in all directions, invests most of the twelve episodes in the girls and their relationships with the coach. In addition, "Softenni" at least is more honest in your intent: fanservice and more fanservice, and, if possible, one or another tennis match. Now "Ro-Kyu-Bu!" masks this content with your innocent visual and lolis characters. Wrong, very wrong...
Both are anime regarding basketball and both are awesome in my opinion. Although, Slam Dunk has less to none echii material (from what I remember) than Ro-Kyu-Bo! which might be less appealing to those seeking echii situations. Slam Dunk is a quite old anime and its drawing style is rather serious but the anime itself is not. Trust me, you'll laugh 'till you feint in some scenes :)
Moe girls doing cute things. Both of these series focuses the will to become an official group that people are willing to recognize in the realm of sports and music. There are some hints of romance in Ro-Kyu-Bu! but both series have comedy and drama.
Well... First things first two of the K-On voice actresses (Yoko Hikasa and Minako Kotobuki) play an important role in the anime which I found quite amusing at first. And the fact that both anime series' revolve around a group of girls trying to put what they love (Basketball and Music) before anything else is amazing. Plus the girls' personalities are awesome in both series :) I realized after watching Ro-Kyu-Bu and K-On that girls who have a lot of passion and dedication can get whatever they hope for.
The vibe is pretty much just the same: loli's here and bishoujos there, harem, and you get these two here.
Seiyuu-wise, Subaru (RoKyuBu) and Yotaro (Baby Princess) are both voiced by Yuki Kaji. You'll also find much Kana Hanazawa in RoKyuBu (Tomoka), and some in BabyPrincess (Mizuki), plus very Kanae Itou goodness, and pretty much Yuka Iguchi awesomeness.
Yuusha ni Narenkatta Ore wa Shibushibu Shuushoku wo Ketsui Shimashita (aka YuSibu) & Ro-Kyu-Bu! are similar in which the main character is given a task in which at start he/she is reluctant to accept but after some persuasion finally agrees to help. The MC in both animes takes it as his/her duty to help one or multiple people over the multiple obstacles that they face. At times when the going gets tough the MC gets tougher and shows that he/she has the determination to see his work to the end and that his responsibility is fulfilled.
The drawing style look pretty similar, characters' behavior and storyline too.
Another common thing is about quantity and age of people, for example, people involved are almost all school friends and their families, and parents of al minors look like adolescents or at least really young to be parents.
The biggest difference is that Clannad is more about the school, while Ro-Kyu-Bu! is more about sports, especially basketball.
Another difference is that main characters in Ro-Kyu-Bu! are younger than main characters of Clannad.
Both animes have basketball centered and have got characters with intense will to play/win. The biggest difference is probably that Ro-Kyu-Bu! has 'young' girls as main characters, whereas Kuroko no Basuke has mostly high school guys as main characters. Both have outstanding art!
Both shows have comedy, great art, the same voice actor for the male lead and a lot of loli fanservice.
HenNeko even references Ro-Kyu-Bu in episode 10, when Tsutsukakushi is throwing away Yokodera's ero magazines.