Random Torchwood thinky thing

Looking at the latest episode re-watch discussion (1x12 Captain Jack Harkness) over on tw_classic got me thinking about how a lot of what is going on behind the series one storylines can be basically boiled down to 'the stupid things people do for love/friendship'

Which then got me thinking if there was a similar one for series two, and I finally decided on 'What's in that past tends not to stay there.'

Series three, 'secrets and corruption make the world go round' or 'power corrupts' or possibly 'there are no real winners in life, only those who lose less'

I've not watched series four, and don't intend to. Does anybody have any suggestions about what that series thing would be?