I honestly don't know a lot about the comic book Thor. I know quite a bit about the mythological one, but I've never really been into the comic books.
So I went into "Thor: Tales of Asgard" with a fairly open mind. This cartoon is a pretty average for its first half (much coming-of-age-and-learning-responsibility plot), and the animation is not exactly great even at its best moments. But the plot is well handled, the characters are well written, and the climactic battle between Asgard, the Ice Giants, and a wild card with an unstoppable weapon.
Teenage Thor is shocked when he's told that he only wins against Asgard's greatest warriors because they have to let him, and that nobody respects him. So after Odin forbids him to go out in search of adventure, he and his reluctant brother Loki stow away on the Three Heroes' ship. Their goal: To find the legendary sword of the Fire Giant Surta, lost in the frozen wastes of Jotunheim.
Unfortunately, Odin soon discovers that not only have his sons left Asgard against his orders, but they're causing lots of mayhem wherever they go. And after finding the sword, Thor accidentally sparks off a war between Asgard and the Frost Giants, leading an army to lay siege to his home city. Now not only must the five travelers find a way home, but they must somehow avert a massive war -- and stop a deadly foe in their own number.
"Thor: Tales of Asgard" isn't the best comic book movie I've seen -- definitely not on the level of most Batman animated movies, or the Green Lantern ones. This is mainly because of the animation, which is very Saturday-morning cartoon. Simple forms, solid sheets of hair, very thick cylindrical calves, and angular figures, and pretty interchangeable faces (especially the women!).
But the rest of it is actually pretty well done. Lots of flashy battle scenes, some clever foreshadowing (Loki's comfort with Jotunheim's weather), and some genuinely dramatic moments (Thor's horror at some of the Frost Giants actually getting KILLED). While the first half is fairly standard coming-of-age story, the second half has a very dramatic, epic feel, which is only added to by the formal language used in the script.
However, there WERE a few things that confused me. For instance, who was that woman teaching magic to Loki at the beginning? It seems like she's going to be a corrupting influence on him, but... she never appears again.
But they do give some very nice character development to Thor and Loki. One is brash, arrogant but good-hearted, and must be taught the true price of his actions; the other is intelligent and cautious, but has a definite dark streak hinting at what he will do someday. The Three are fun, Sif is a pretty standard love interest, and Algrim is a tragic, painfully dignified figure who can't quite quell his hate. The scene where he goes berserk is a bit silly, though.
"Thor: Tales of Asgard" isn't a great animated movie, but it's a pleasant way to spend a few hours. Lots of action, a likable hero, and some powerful character development.