You know I don't like action movies, and I hope nobody is surprised that I didn't like Ninja Scroll very much. All the positive reviews I've read seem to find it good mostly because it's the first anime the reviewer has seen. Sorry, I've been watching anime since 1982, when I was five, and Ninja Scroll is really nothing special.
The story: the Toyotomi family hires a group of demonic ninjas to retrieve a large amount of gold, to be used to wrestle power back from the Tokugawa (which dates the action between 1600 and 1615). An elderly ninja working for the Tokugawa tricks the main character, Juubei, a mercenary, into helping him figure out the whole story. Super-powered fights ensue, an old enemy (presumed dead) resurfaces, all the plots fail spectacularly, the gold is lost, the power structure is untouched.
Nothing particularly new there: ninja stories were very well established in manga and anime, a good example would be The Legend of Kamui (カムイ伝) by SHIRATO Sanpei (白土 三平) was published between 1964 and 1971, and transposed into anime as Ninpuu Kamui Gaiden (忍風 カムイ 外伝) in 1969. The supernatural, super-powered aspect is not very new either. The story progresses just like video games had been doing for decades, with the enemies attacking one by one, even when clearly outmatched, and the boss fight at the end. The characters are very stereotyped, probably to save time building them, and use it on the action scenes instead.
I've read that many consider the animation to be very good. I'll just point out that Ninja Scroll was produced in 1993; Akira came out 5 years earlier; the two Patlabor movies are from 1989 and 1993; Miyazaki had already directed Castle of Cagliostro ('79), Nausicaa ('84), Laputa ('86), Totoro ('88), Kiki ('89), Porco Rosso ('92). So no, the animation is not above average for a movie of that time.
The director, KAWAJIRI Yoshiaki (川尻 善昭) was already an old hand at action movies: he had directed Wicked City ('87), Demon City Shinjuku ('88), Cyber City Oedo ('90), even a Lensmen adaptation (SF New Century Lensman, SF新世紀 レンズマン, '84). Interestingly, he had also directed the third chapter of Kadokawa's adaptation of Tezuka The Phoenix ('87), which is definitely not an action-based story. In Ninja Scroll he shows very well that he knows what he's doing, but doesn't really shine.
One interesting details that I liked: there is at least one homosexual and one bisexual character (Yurimaru and Genma), and this is treated as completely unremarkable.
In conclusion, this movie scores high on the "action packed", "gory", and "nostalgic (for some)" scales, and I don't doubt that many people really like it. It's just really not my kind of product.
