I'd say it's due to the huge amount of effects work a lot of the episodes will contain. Torchwood will contain some CG, I'm sure, but it won't be anywhere near the scale of Who. Also, they DID have problems with the HD for Torchwood which probably resulted in the decision not to use HD for Who series 3.

What I want now is for series 4 to be a complete revamp of the show. With HD coming in they should take the opportunity to change the way it's shot - get rid of that soft vaseline filter FOREVER, tone the bright colours down and make it look less like a CBBC series. Also, the music should get a facelift. I know some of Murray Gold's electronic crap annoyed me, but if it's done right (probably by someone else) it would suit the series more than the overblown orchestra stuff. The orchestra seems too big for what's on screen. It's almost like the equivalent of someone mixing the Lord of the Rings soundtrack with Hi-8 camcorder footage of their back garden... The title sequence and credits should also be revamped. 3 years with the same will be OK.

I wonder if Tennant will still be there for series 4, or RTD for that matter. Who the heck would take over Davies's job? Steven Moffat? And if Russell is still there, which lead actor that he's already worked with will be the 11th Doctor? The Second Coming was Eccleston's audition for the 9th Doctor. Casanova was Tennant's audition for the 10th. So who does that leave? Er, there's Alan Davies who was in 'Bob and Rose', Griff Rhys Jones(!) from 'Mine All Mine', there's a couple of guys who were in Queer As Folk that would probably be good, and Barry Chuckle (I think he wrote a couple of episodes of Chucklevision, didn't he? or was that just a joke).