DBZ is the anime that gets many fans into anime. Think its it is actually pretty poor, just addictive. Most anime fans then evolve from DBZ onto more classic animes like Eva, retro like Dominion Tank Police, or modern anime's like Ghost in the Shell, Mezzo etc
Personally I don't hate DBZ, although I do now share the same criticisms of it most other anime fans do when they're been watching it for 6 years and have seen much better titles.
However I do not hate DBZ, because it is the anime that gets many westerns into anime, and has given me some lasting memories.
The problem with is not with 'true anime fans', personally I'd see this as anyone who watches anime on a regular basis. Nor is it with Otaku's who I'd class as anyone nuts about anime enough to buy hundreds of DVDs and watch it constantly....... that would be me

. The problem is with the Purists.
Purists tend to have a common theme, that being most have been watching anime for decades, usually online or in small fan clubs. These titles were always in Japanese until they became mainstream and dubbed. They were always small in number.
Purists liked this feeling they were special and unique, it was a cult thing, and they where are its heart.
However, as anime became more mainstream, it lost its cult status as more and more people started watching it. Especially animes like DBZ, which are watching by millions of people in the west.
Purists tend to do little but complain about how dubs are a rape of Japanese culture (no matter how good ADV has become at dubbing) and wishing it would return to the days of them in their dark rooms watching retro anime on their computer screens.
Since I'm not a purist I can't tell for what exact reason they hate DBZ more than it deserves, but I think I can place it down to two reasons;
1. It was essentially the anime which made Anime mainstream in the west, more-so than Akira and GITS, and I think they hate it for doing that to their cult, and in their eyes turning anime into a 'fad'. And lets face it, purists are very vocal in calling anyone who's only started watching anime in the last 10 years a hitchhicker who'll leave anime once something new is around. This is rubbish of course, I started watching anime in the last 6 years, and I think I could easily beat half the purists in the quality of my collection and time spend watching it.
2. I think they hate the definition DBZ gave to anime for newbie's and outsiders. Every decade in anime seems to be defined by particular animes. In the 1980s as a child I remember seeing adverts in game magazines for old manga titles like Project A-ko, Street Fighter II and Dominion Tank Police (got them

), both rather weird dark animes. The era meanwhile was defined by Legend of the Overfiend, a hentai anime which was so infamous everyone knew about it and for many it was the definition of what an anime was even though only 10% of anime titles are hentai, thus anime was rather taboo.
In the early 1990s the purists rejoiced as this crown changed to Akira. A dark sci-fi anime (which personal I hate) which they felt was a far better standard for their hobby. Life was good.
Then in the late 1990s that was all shattered by the sensation of DBZ. Suddenly anime for many people wasn't just a much hushed taboo media format that you must hide your children from. Suddenly it was a colourful, action packed young people's fighting show on every cartoon station in America and Europe. DBZ lacked a good plot, it was horribily dubbed and did not show any of the wit and style of other animes such as Eva (which at the time was the anime all the hardcore, otaku's and purists were watching and wanting to take Akira's place). But to their horror Eva was to remain a title only really familiar with anime fans, while DBZ became the anime everyone outside the hobby knew about.
Unfortunately for us all, DBZ was then replaced by........... Pokemon

........which was then replaced by Bayblade

And thus the perception of anime going from adults, to teens, to children's programs was complete. And Purists hate DBZ as it was the first anime which took make the outside world stop seeing anime as mature content.
Personally as a hardcore anime fan, but not a purist, I do not see the threat of anime becoming mainstream. I actually like dubs, and I want anime in the western market.
As much as I do DL a lot of torrents, I do prefer to be able to watch anime in a language I understand from a nice shiny disk.
6 years ago when I started collecting the only anime’s I could find in the shops were Eva, Giant Robo and various…. Very dark, titles released by Manga (why on earth was nothing happy ever released back then?

). There were only about a dozen titles on the UK market. Now anime has become mainstream, a dozen new titles are being released over here every month, meaning my collection no longer relies entirely on US imports.
Personally I like that I can get more anime, personally I like there are more fans to talk about it too. Purists are just sad old gits would can’t get past the old days are over. Yet very hypocroitically they ignore the benefits of a larger anime fan community. Big O was a flop in Japan, and would never have gotten as second season if not for the popularity in the US. The western fans are also the most vocal in demanding a second season of Cowboy Bepop, and although its unlikely ever to happen, if it did I would think the constant nagging of the west would have had some impact on it.