You forgot Amazon, whose service is actually a better deal right now, and if their database gets built up, will be a huge competitor to Netflix.
Also Facebook is playing with the idea of streaming movies. Oh yes, have we mentioned the usage caps by AT&T?? There are just so many things that can go wrong with Netflix, and I don't even see how they make good money with all their costs.
I can get 6 movies a month (their return is fast), that is 6 x 80 cents, $4.8 just for shipping out of the $12 monthly fee. (they introduced streaming only options for a reason) Then throw in copyrights, CDs etc. and their profit is disappearing.
You mentioned Redbox, and if you don't stream movies, that is actually a better deal, because you get 10 movies for $12 and you don't have to wait for it in the mail.
So Netflix's possible problems are:
1. Increased competition from other online streamers.
2. Usage caps from ISPs.
3. Other cheap movie distributors, like Redbox.
4. Movie studios want a bigger share.