Comcast has begun compressing HDTV shows in order to deliver more HD channels to you while using the same amount of bandwidth. They didn't use to do this before, but now, when compared to Verizon FiOS, the channels are grainy and blocky and full of artifacts—a result of shoving three channels into a space where only two previously occupied. A guy at AVSForum measured how the new bitrate stacks up against Verizon. 
The forum poster says that the compression isn't too horrible with still images, but gets really bad when stuff moves around.
The greatest differences are seen with movement. With slow movement on Comcast, the first thing you notice is added noise and a softer image, as fine detail is filtered from the picture signal. The greater the rate of movement, the more detail you lose and the more noise you see. With intense movement, you see more blocking and skipped frames. In VideoRedo, I noticed that a number of frames in the FiOS signal simply did not exist in the Comcast signal during motion intensive scenes. This may be responsible for the stutter and excessive motion blur seen with some video sequences on Comcast.To Comcast's credit, I saw little to no difference on movie channels such as HBO, Cinemax, and Starz. I did see some blurring and reduced detail during fast movement on Starz, but the recordings from Cinemax and HBO were virtually identical, even on action movies such as 300 and Gladiator. When there was blocking on the Comcast feed of Cinemax, that blocking was also on the FiOS feed.