I think a lot of people like it because it's very accessible, just as DBZ was. Any show that consists mostly of characters fighting one enemy/rival/whatever after another are easy to pick up and start watching because although plots are present, no prior knowledge of the plot is required to enjoy watching. This is because watching some dudes beat up each other can be pretty entertaining whether you're knowledgeable of the series or not. Also, the visceral enjoyment from the fight scenes will last with the viewer, and as a result he/she will be tempted to delve deeper into the "universe" laid out in the anime/manga/whatever. Pokemon also falls into this catagory, as it follows a very basic formula as well.
There's also the "bandwagon" factor, which is just as responsible for people NOT liking the show as it is for people liking it; there are those of us who will "hop on the bandwagon" and enjoy the show because everyone else is, and then there are the people who will resent it solely because others like it.
I find that it's a pretty average series. I watch it sometimes when I'm at a friends' house who likes it, but I really can't get into it as it seems pretty repetitive and I don't particularly care for Naruto as a character. There's so much of it, and so much of it is essentially the same, and there are so many irritating rabid weeaboo fanboys/girls that love it, I don't like it much.