This series has become very, very popular. That means it has attracted some radical fans, and so some of the more moderate ones love to find things they hate about it now. Fans ruin everything...
Plot: The plot's a pretty typical coming-of-age sort of thing.
Length: LONG. There were some things that could have been greatly shortened. Like almost every arc, actually.
Characters: Kishimoto did a good job with this, actually. He populated his world with a lot of interesting characters. Somehow, he managed to make even minor characters memorable. Each person got their own unique traits/powers that made them stand out from the others. I think most people who have read this will say that the main protagonist (the title character) is not their favorite character. Just look at how many times characters like Hinata and Itachi, who really don't appear that much, have placed on the top ten of character polls! Kishimoto gets +10 for characters.
Art: The art's all right. It's not amazing. However, it has noticeably improved as the series has gone on. There are some places where the art is better than normal, and the heightened quality adds to the story.
Possibly Offensive: Things can get quite violent. When people attack you head on in this story, they seem to go right through you---literally. Like through your hand (Naruto v. Kabuto), or your shoulder (Naruto v. Sasuke). Or your chest (Tsunade v. Orochimaru). Actually, the character Orochimaru himself should fall under the 'possibly offensive' category. He's a good villain, don't get me wrong, but sometimes he made even me shudder. There's really not that much fanservice. I think we females get more fanservice thrown our way than the males. Karin wasn't the only one fangirling in some of those shots where Sasuke was half naked.
Other Stuff: This series really isn't too deep, but it's been entertaining me this long, so I'll give credit where credit is due. Sometimes Kishimoto throws out some real wallbangers, but then other moments are very touching and poignant (the best example is the fight between Naruto and Gaara). It's when those moments come that I remember why I keep reading this series.