In the time before time, there was an anime called cutie honey... I'm kidding.
This is probably the oldest anime I ever saw. It aired in 1973 (nineteen seventy three). As a result it is pretty much unlike anything I've ever seen, and the closest thing to it, at least visually, is the first season of Sailor Moon. The anime is also based on works on Go Nagai, who is responsible for Devilman, Devilman Lady, Getter Robo, Mazinger, and many other things.
The age of the anime is noticeable in every its aspect - story, characters, visuals, art style and music. It aired before anime became very popular and well kknown, and as a result it looks much simpler and much more naive than ANY modern title. The art style is quite different as well, with drawings being flat, and backgrounds being extremely abstract and colored in manner resembling old cartoons like Woody Woodpecker or Pink Panther.
It is still a very nice anime, in the end, despite silliness, strange (by modern standards) style, simplistic story. The music is nice as well.
Detailed breakdown below:
Story:
The story is very simple and strictly follows "monster of the week" pattern. It also holds world's record at fastest character introduction - it managed to cover honey's backstory in 5 minutes or even less. Haven't seen anything like it. The story revolves around honey fighting a criminal organization, which wants a certain item Cutie Honey has, and every week sends another female monster after it. The monster always is some sort of hybrid creature, always female, always called "something - CLAW". The structure of every episode is the same every time as well - introduction of the monster, fight, followed by iconic transformation sequence, and brief wrap up. Overall the whole thing almost looks like a kid's movie (with a script written by another kid as well), that is until heads start flying.
In the same naive fashion anime displays its characters, it easily introduces dismemberment and occasional death. Some deaths are quite gruesome, and after happily jumping around in another costume, cutie honey may lop off someone's head or limb with her sword. This doesn't happen often, mind you, but happens. Which reminds the viewer, that, in the end, it is an anime, and not a western cartoon.
The story does not really have an overarching plot, and largest portion of it could be thought as more or less independent episodes, if next monster weren't introduced at the end of episode, although things change a bit at the end of the series.
Locations, circumstances, people, interactions are simplistic, and remind of a kid's movie, and as a result are not very "believable", but still fun to watch. In the end the story is mostly about Cutie Honey prancing around switching costumes and killing bad guys.
Art:
Art is unusual by modern standard. Characters are very "flat", meaning their visuals look like something half way between modern anime and, say, old cartoon like Woody Woodpecker. They don't really move about well, deform during movement, and anime does not really portray "depth" anywhere.
The backgrounds in scenes resemble abstract painting, or old backdrops occasionally used in western cartoons. Pink Panther or old Cartoon movies. Basically, it is collection of shapes, usually drawn using flat color. Basically, coloring of backdrops is not dissimilar to, say, south park, although, lines in backgrounds are fluid. Hopefully it makes sesne.
Most of the mechanical objects are not very well drawn, and barely have any detail on them. Main character - Cutie Honey - is amazingly cute, but some of the other characters are not. One of the side characters have a face that looks like a.. an undead peanut. Can't describe it in any other way. "Bad guys" always have evil face, fangs, "evil grin" and slanted eyes. Very different from modern anime.
I would say that overall drawing style shares some similarities with very old titles - for example, it may slightly resemble the very first episode of Urusei Yatsura, and the very first episode of Sailor Moon... I'd say more similar to Sailor Moon, which had "swirly glasses" character.
Sound:
Music is amazing and catchy. Original intro is something you'd want to sing along to ("Kono goro hayari na onn-nano-ko...") and will get stuck in your head. There's also a constant "whispering" theme that usualy plays when an episode displays this week's villain. (you'll recognize it when you hear "Shi.. shi, shi, shi- shi... SHAAAA!" in background). The anime doesn't have too many soundtracks, however, and seem to be reusing about 5 tunes total.
Characters:
Characters are pretty much symbols. They have specific role or function and are completely defined by it. Cutie honey's role is to kill bad guys and look cute while wearing outfits, for example. They don't really develop anywhere, and don't have much of personality to display, they pretty much only do one thing they were designed to do. Honey occasionally tries to act seriously, but it always comes off as awkward and strange.... then again it is sorta in line with "naive old anime" feeling about it.
Enjoyment:
It was an interesting and a very refreshing experience. The anime is unlike pretty much anything that is released nowadays, and it was nice to see something from time when plots were simpler, grass was greener, etc.
Overall:
I recommend to check it out, if you have opportunity. For historical opportunity, or to see what anime used to be in 1973. However, if you're looking for anything serious, the anime might not be the right choice, becasue despite its occasional attempts to have serious or dramatic moment, it may look silly... but then again, it is Nagai GO material, and some of his older works may come off as a bit direct/naive by modern standards.
So, for something more serious, a good candidate would be original Sailor Moon, which starts off with similar formula and somewhat similar old art style, but then becomes much more dramatic.