Not to be argumentative with anyone, but here's my take.
The beauty of downloading movies is that you don't have to go to a video store or wait for the mail (or UPS/FedEx) to deliver you a disc. While not instantaneous, d/l movies means you don't have to worry about the store (or Netflix) not having the latest release because everyone else in your area wants to watch the same movie. Plus you don't have the hassle of having to return the disc.
As for downloading speeds, yeah, right now it's a bit of a downer. TV shows and iTMS movies take a long time for most people to download. It'll get better eventually, but it's probably the biggest "minus" for downloading movies.
Rental vs. Buy - I prefer to rent. I will typically watch a movie once or twice and be done with it. Certain movies, however, I want to own. So I'd like to see both options available.
HD vs. SD - not everyone will admit it, but HD is coming and it's coming fast. I had been holding out for a long time, but we finally took the plunge to HD and it's awesome. The only downside is the lack of HD programming for cheapos like me. I currently only get the local HD channels and UHD from my cable company. I got a TV that has a QAM tuner, so I pick up any digital and/or HD channel that isn't encrypted. No extra charges for Digital Cable, cable box, etc. So that means 8 HD channels only. I'd love to have downloadable HD content, even if it's only 720p compressed to 5GB (for 2 hours).
X-Box vs iTV - I like both. I like the iTV based on what's been leaked/disclosed. The ability to stream media from my Macs is great. I don't have any iTMS movies yet, but I'm guessing it would look like crap on my HDTV. The X-Box 360 with the d/l HD movies sound great, but I was really looking at getting a PS3. Maybe Sony will jumble up the mess with their own Movie Store for the PS3. I think this will all shake out in the end and my vision is that everyone will offer HD Movies and TV Shows for downloading and that movies will be for rent/buy and TV shows for buy only.
ft