Well the critics really seem to like it. They're always talking about how "adult and edgy" it is. Do the DW fans just not care for it because there's not enough scifi in it or something?I haven't seen it yet myself; I'm just curious.
Actually, I've had it pointed out that there's a lot more SF in Torchwood than in Doctor Who, which is often seen as "science fantasy" by some. And no I won't restart that old debate - there's a thread about it over on SF/F if you want to dig in.
You'll get as many different answers to your question as you'll find DW fans. Speaking as someone who enjoys Torchwood greatly, I've noted some of these factors in reading some of the hatred towards this show (note: I differentiate between hatred and criticism):
* Russell T. Davies created it. Never mind that RTD made Doctor Who relevant again, award-winning, a pop-culture phenomenon, hired David Tennant, hired Steven Moffat and gave the OK for stories like Blink and Girl in the Fireplace, etc, etc, he's considered the Rick Berman of the DW franchise by a lot of people. Meaning anything he touches is automatically considered shit by some people.
* People have sex. Maybe it's a holdover from the "SF is for kids" stereotype that might sit in people's subconscious, exacerbated by the fact Doctor Who is considered a kids show in some quarters, but there's this automatic assumption that any hint of sex or romance is simply not to be tolerated in this venue. Just look at the conniption fits that happened when McGann kissed Daphne Ashbrook in the TV movie. And you'll find plenty of people complaining about how Rose and Martha were lovey-dovey over the Doctor. And we won't even go into the nuttiness that ensued when Amy snogged the Doctor and tried to go to bed with him. And no one even had sex in those instances. In Torchwood, people shag. (Star Trek fandom also has major issues with sexuality in the TV series and films. Yet many of these same people could care less if Aeryn and John were to get it on every other episode of Farscape, or the cast of Firefly rocked the van, or Starbuck slept with half the deck crew on nuBSG.)
* People of the same sex have sex. There's not much can be said about this one. Some people accept it. Other people are offended. To each their own. But it's definitely a factor.
* John Barrowman feels threatening to some people. I had prepared an explanation of that one, but I figured it was just flame bait to I took it out. Basically if the statement above applies to you, then you understand. If it doesn't apply, then you won't.
* The series stumbled a couple of times coming out of the gate. Hey, the last time anyone attempted a spinoff of Doctor Who they gave us that awful K-9 and Company. TNG, the first Trek spinoff, took nearly 4 years to find its footing. The first season of Torchwood was as much about experimentation than anything else.
* Martha Jones in season 2. Nuff said. Some people just don't like that character.
* Deaths of certain characters. This more applies to seasons 2 and 3. There's a reason why Doctor Who rarely kills off companions. In Torchwood, with the sole exception of Jack, you really don't know who'll survive the season.
* Dark-i-fying certain aspects of DW canon. I know a lot of people were pissed off to see UNIT depicted as running a Gitmo-like establishment during the second season.
* Since you haven't seen it, I can't say much that won't be a huge spoiler, but the Children of Earth mini-series (technically the third season) breaks pretty much every rule established by Doctor Who. While I am of the opinion that this is why Torchwood exists -- to tell stories that cannot be told on Doctor Who, full stop -- others don't agree and feel the spin-off should follow the same rules as the parent program. Sort of like how many many many Trekkies were (initially, at least) upset when DS9 broke the mould and delivered a darker, more morally ambiguous series than TNG and TOS. Yet by the time it ended, it was hailed as a classic with many naming it their favorite Trek series (I'm one). Torchwood has the potential to do the same thing -- indeed I'm of the opinion that Children of Earth was, in many (not all, but many) respects superior to any Doctor Who produced since the series returned.
Alex