Can't really tell the difference between 720p and 1080p from my computer screen so 720p is good enough for me. Sometimes I download the 1080p version if the file size is not too ridiculously large.
For Anime and TV shows, I generally download 720p. Mainly because, for some groups, the 720p releases first. Also, with anime at least, the difference between 720p and 1080p isn't too noticeable, due to the nature of animation in general.
Same here, but not because of the size, but it puts a strain on my eyes when I watch in 1080p. Might be because I watch on my PC, but, yeah, that's the main reason.
I don't know; I think a lot of recent titles actually look good @1080p . Although I understand it might not look that way for everyone especially if you are watching in a relatively smaller display.