[KPackratt2k]

It depends on how the provider/distributor encoded the program and the frame rate it was produced in. In my personal experience, I bought some Disney Channel Original Movies and shows from iTunes solely because they didn't get released officially on DVD, for most of them, the image quality looked about as good as if it were to get an official DVD release, but for a few other titles there were noticeable compression artifacts that made it look worse than what you'd expect out of a DVD, so much so to the point where even a good HDTV rip had less compression. For example: the first Zenon movie had some noticeable color banding in its iTunes version.

As someone already briefly mentioned, digital services do change the frame rate on interlaced content, in my case these older (SD mastered) TV movies were IVTC'd (converted from 29.97i to 23.976p), so scenes that were fast motion when these movies previously aired on TV didn't feel quite the same as they used to (you can easily spot this change in scrolling end credits). There was also noticeable deinterlacing in a few of these movies as there were jagged edges.

Another compromise that might be bothersome is the way the audio is compressed. More often than not, digital services (including streaming and purchases) use AAC compression at 128kbps for Stereo sound, whereas DVDs typically use AC3 compression at 192-256kbps for Stereo (384-448kbps is more commonly used for Surround sound than it is for Stereo). I've also noticed that the audio sample rate of most of my iTunes movie purchases was 44.1KHz (CD sample rate) rather than the 48KHz sample rate that DVDs commonly use.