Children of Earth is actually pretty approachable if you haven't watched previous DW and TW; hobsonphile watched it without any previous Whoverse knowledge, and it worked for her. The only continuity-relevant thing you need to know is that we find out in the Torchwood pilot that the Rose/TARDIS amalgan in Parting of the Ways didn't just bring Jack back to life, she made him immortal. This... is something of a mixed blessing, to put it mildly.
(If you're curious, the correct timeline for New Who and TW is:
New Who s1 and s2 - TW s1 - New Who s3 - TW s2 - New Who s4 - TW s3. So when you see Jack again in DW's third season, the s1 finale for him has just happened, when you see him and the other Torchwoodians in s4, they're past the s2 finale and before s3. But though there are two dialogue references to the Doctor in Children of Earth, they're not spoilery for DW in the slightest, so don't worry.)
As for why the people who hate it, hate it: wellllll. Your guess is partly correct. The other reason, imo, is the very one that made people who'd given up on the show in s1 change their minds. To quote nomadic_writer who commented to my review for Day Four : See, I'm only a very casual occasional Who-watcher and have never liked Torchwood at all, so having caught the last three episodes of this miniseries, I have a question: When did this show stop being all stupidity and inappropriate sex talk and suddenly become brutally hard-hitting and awesome? Because that was kind of a surprise.(What was not a surprise: checking friendsfriends for reactions and discovering that just when I thought it had got really good, ninety percent of the fandom hated it and declared the show ruined forever. Ah, my old friend the minority opinion corner. We meet again.)
And astrogirl2: man, it's really nice to know that I'm not the only one sitting here going, "That was awesome!" rather than, "NOOOOOO! RTD, I AM ANGRY AND I HATE YOUR FOREVER!" :) And an earlier comment of hers: I am very much enjoying the new Torchwood. Somewhere along the line, Torchwood stopped being mock-worthy buffoons and started being complex, competent, brave people I could really root for.
Rewatchable: I rewatched it four times already, if that answers your question...
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(If you're curious, the correct timeline for New Who and TW is:
New Who s1 and s2 - TW s1 - New Who s3 - TW s2 - New Who s4 - TW s3. So when you see Jack again in DW's third season, the s1 finale for him has just happened, when you see him and the other Torchwoodians in s4, they're past the s2 finale and before s3. But though there are two dialogue references to the Doctor in Children of Earth, they're not spoilery for DW in the slightest, so don't worry.)
As for why the people who hate it, hate it: wellllll. Your guess is partly correct. The other reason, imo, is the very one that made people who'd given up on the show in s1 change their minds. To quote
And
Rewatchable: I rewatched it four times already, if that answers your question...