The setting is a future where Japan has a declining population and schools have less students partially due to the lower birth rate and partially due to a system of apprenticeships which leads a lot of youngsters to just skip school altogether. The setting is Seioh High School where an incredibly energetic transfer student bursts into the school and quickly nominates herself for student council President, which she gets because no one else wants the job. The episodes follow her and four other girls as they attend to student council business, in spite of several of them not even being part of the student council. The episodes do have a clear progression but they don't have an underlying story. It's reminiscent of Azumanga Daioh in that respect. There are a few major themes, mainly camaraderie and moving boldly into the future, personified by Manabi's favourite saying "Forward Go!" The major difference between Azumanga Daioh and Manabi Straight is that Azumanga Daioh focuses on the comedic aspect and Manabi Straight focuses more on cute, fluffy moments. On the plus side, it works very well. It's a fun series with lots of good and funny moments. The themes get a little heavy handed, but they're pretty well handled overall.
By themselves, the characters are pretty standard archetypes. However, they interact very well, they play off of each other pretty strongly and they bring out complexities within each other that aren't readily apparent. I can't really tell if that's a deliberate way of strengthening the camaraderie theme or if it just turned out that way. I have a feeling it's the former.
The art is the one aspect I really don't like. They use a super chibified art technique that makes all the characters look roughly ten years younger than they're supposed to be. The imagery is very deliberately bright and vibrant. They also do some really interesting things with the art at intense moments. Sometimes they go overboard, but it usually works and, in spite of my general dislike of the art form it works for the series.
The voice work is pretty good, for the most part. There is some over-exaggeration, especially from Horie Yue but it generally works for the scene so it's not too much of an issue. The music is spectacular. The school anthem, the themes, and the background music are all perfect for the series and excellently performed.
The yuri factor is a 6/10. The main five girls cuddle up to each other consistently. Mikan also develops an obvious crush on the student council president of another school. None of them go beyond being cuddly, but it's still pretty homoerotic and there's definite yuri potential.
My final rating for Gakuen Utopia Manabi Straight is a 7/10. It's a good series. If you're looking for something that's energetic, funny and just generally fun you'll probably enjoy it. It's kind of similar to Azumanga Daioh or Gokujou Seitokai, so if you're a fan of either of those I would check it out. However, if you're looking for a complex story or action-packed fight scenes you should probably look elsewhere.
As for why it was never released outside of Japan, maybe they felt that an anime that emphasises cuteness and uses humour as a secondary aspect wouldn't do well for international audiences. That's my best guess.
