With the Alien movies, you can buy each movie individually on DVD, but on Blu ray, the only option is all four of them together. I loved the first two movies, liked the third, and despised the fourth.
Same with Star Trek The Original Series. I've seen those shows sooooo many times, and I might like to get a few favorite episodes on DVD (you can get two episode sets), but on Blu ray it's just season sets. I can't see spending the money for a whole season when it contains a ton of episodes that I find fairly mediocre that I've seen 10 times each and have no interest in owning.
Also with Star Trek, I'm quite confused about this whole added CGI and remastered theme song stuff. On the DVD season sets you can choose the original theme song or the (god-awful) remastered one. Even after reading entire in-depth reviews, I still can't figure out if you can get the original theme on the Blu ray. Good grief, like somebody nowadays could "improve" on music made in the late '60s