Saint Seiya is a shounen that features several elements of mythology (mostly Greek mythology) and astronomy (constellations). The main characters, the Bronze Saints (knights), are responsible to protect goddess Athena and the humanity against the evil gods.
The fights are generally interesting, specially because the majority of the characters have a badass aura around them. On the other hand, they are more static than in other anime of the same genre, mainly because of the few techniques used by the Saints/knights.
The story is quite simple for the most part, and goes in a very monodimensional way, but it is solid.
Although the main focus of the plot is the war between Gods, the plot of the first arc revolves around the consequences of the Pope’s betrayal in Athena’s Sanctuary.
The first episodes feature the Bronze Saints fighting each other in the highly used but never old “tournament system”, competing for a very desired prize: Sagittarius Gold Cloth.
After the introduction of the main characters, Sanctuary arc really starts. We are introduced to a corrupted Pope, a man who should profess Athena's wishes, but uses Athena's soldiers to his evil intentions.
The Pope sends the Silver Saints to kill the Bronze Saints with the excuse that they deviated from their path. Some of the Sanctuary Saints know about the Pope’s evil intentions, others not.
It’s valid to say that it takes some episodes to reveal Athena's true identity. The same thing for the Pope.
It also takes some episodes for the Gold Saints appear. They are the strongest Athena's soldiers.
After this small summary, I tell you that Saint Seiya is a very entertaining anime, but it may have not aged well for the newer audience. I really don't now if it will please people that are only into modern days shounen.
Also, as I said, there are a lot of faults in the anime story when you compare it with its manga. So I wil tell some of them in the next lines.
Differences between anime and manga
The anime has some features that are superior than the manga, like the art in general, especially Character Design and Clothes (armors).
But the studio also created a lot of filler characters: some of them has not even a category (Bronze, Silver or Gold) or a constellation well defined.
There are also the total unnecessary Steel Saints, that have good-looking armors, but fight just using mechanical weapons, without cosmo (chi). They appear in some episodes at the beggining, but then they vanish without any explanation.
Other big difference is that Hyoga is disciple of Camus in the manga. In the anime, Hyoga is disciple of Crystal Saint. And Crystal Saint is disciple of Camus. This will have a significant implication in the future arcs (reveal more than this would be a spoiler).
The 12 Zodiac Temples Arc starts well, following the manga as much as possible, but the final fight is very different.
After this part, there is a whole filler arc, the Asgard arc. It features new enemies: the God Warriors. The armors and personalities of the enemies are very well made and solid for a filler. But fillers always have some fault: the God Warriors were made so strong, that they somehow overshadow the power of Marinas (from the following saga, cannon material).
This anime ends in the Poseidon arc, and has its sequel in Hades Chapter OVAs.
Animation and sound
Saint Seiya's songs are very, very good. The quality of the animation decreases in some episodes but it can be considered generally satisfactory if you take into account the time it was made. The character design, as I said, is way better than in the manga, especially the Clothes (armors).
Though some people may criticize their lack of free will in following Athena, I think the main characters of Saint Seiya are very catch. Each one of them helped to create the stereotypes of shounen characters (even influencing the conception of Gundam Wing's main cast):
Pegasus Seiya (impulsive and obstinate); Shiryu (calm and rational); Hyoga (sometimes hesitant, but rational when fighting seriously); Shun (kind-hearted, peaceful and emotive); Ikki (antihero, independent and secure).
All this characters are orphans who grew up together. But the singular relation between them is better justified in the manga, where they are all brothers.
Other saints I must highlight are the Gold Saints. Each Gold Saint represents one of the signs of the zodiac, sometimes even in powers and personalities, though some of them must be a little stereotyped.
Most of the enemies in the series are fairly developed for an 80's anime, as they usually have a background story and a meaning to fight that takes the story beyond the good x evil plot.
I am suspicious in praising Saint Seiya, since it was my first anime, so it was very important to my childhood. But I can recommend it to those who like old-school anime, since it may be dated to younger viewers. I also recommend to read the manga, if you have time and want to better understand the story. The art is worse, but the plot works better.