It's the resolution between the two. The resolution on the blu-ray is significantly better. While watching the Torchwood blu-ray dvd's you can see John Barrowman's pours.... Sometimes this can be a good thing other times it can be a bad thing.

Cnet has a quick about blu-ray vs dvd (http://reviews.cnet.com/4520-13817_7-6462511-2.html)