American animation is of better art quality, lots of different styles and methods. Love it. Most do tend to have stupid plots though since the majority are aimed at kids and kid shows in America are over-censored nowadays, but they aren't all bad.
Japanese animation is produced quicker, so there's a lot of cut corners, the backgrounds are often traced/copy-pasted and the character art tends to be weak frame by frame, they all seem to use the same method and just different variations of the same style. It gets old. That said, the stories often tend to be more "grown-up" which is nice. I'll ignore a crappy art style so long as the story is good.
If I think about it, they probably produce the same amount of crappy versus entertaining shows on average.
Only the American ones seem to really only be entertaining by being "stupid funny", whereas Japanese animation actually pulls off real action or horror more often. Both can be entertaining when done well.