I just feel like the other characters and the plot are a bunch of chess pieces in play to affect and effect Jack and Gwen. I guess that's how Doctor Who is too but it's a bit more well rounded and so far the growth as actually carried forward in a consistent matter.
I'd deviate and say that Owen and Gwen were the only characters I felt the writers took an interest in developing.I mean I never warmed to Owen, but the episodes that weren't focused on Gwen were usually focused on him and even in the ones that didn't he was fairly prominent and I felt by the time he was killed off I knew him fairly well. Jack was obviously a main character because of John Barrowman, but even he kind of stagnated for me considering the things he supposedly went through. He always felt like a character that was there mostly for Gwen to react off, to be the ying to her yang, sometimes absurdly so, and I never really felt much growth or after effect from him. Even his story lines started to feel like they were stuck in a pattern. I feel like Tosh and Ianto were the least fleshed out of all the characters, including guest characters, but even they I felt I understood better than Jack. I always felt, barring a couple of episodes that the guest characters of the episodes were better developed and fleshed out than the main characters tended to be.
It does seem like they wanted Torchwood to have the same character structure as Doctor Who with the main focus on the Doctor-like Jack and his companion Gwen with recurring others. But I think that worked on Doctor Who because the Doctor and his companion(s) move from place to place and really are the only characters that appeared regularly. Whereas Torchwood involved a 'team' we supposedly got to know week after week. That creates a different dynamic, but I don't think the show ever really came to terms with that. Might explain why they killed off all the other characters in quick succession and went back to the drawing board. S3 and S4 seem to work more like Doctor Who now where Jack and Gwen are the only real consistent and the new characters aren't really designed to stay for longer than a few episodes at most.
Watching TW straight through was kind of a head spin having the characters fall back into their place with not a lot of growth. Each episode was almost stand lone and most of the character movement reset at the start of a new episode.
I think that's what ultimately turned me off the show. They'd go through these big emotional upheavals or events or characters would be killed off, but by the next episode/ season it would be forgotten/ not amount to much unless it happened to be directly connected to the new plot. Even stuff like injuries never carried over either. Gwen is shot in Countrycide and Ianto is beaten to within an inch of his life, but you wouldn't know it in Greeks Bearing Gifts even though it's clear not much time has passed between the two episodes. I think Burn made an effort at the beginning of EOD to remember that Owen had been shot by Ianto in the previous episode, but that was pretty much it. Or they would show some growth only to have it retconned by successive episodes. It just became harder and harder to care about all their drama because it was almost always about the immediate dramatic moment and not much else.