Or deep could actually be a judged or regarded thing, and not an opinion-based way to say you like something. You can like something all you want, but it can be shallow as hell, just as you can hate something all you want but it can still be deep.
Deep can relate to a bunch of stuff, among them being character development or plot. Things try to be deep (not in the sense of "lol this anime is trying to be so deep) in trying to get one of those...areas of deep or...whatever you wish to call it involved in you more than "flashing images in front of my eyes are cool." Whether it tries or fails, is up to opinion.
By that rote though, you can reduce anything to "I liked it"/"I didn't like it" which would pretty much make for boring discussions or pointless ones. It, essentially, makes every post one of those two positions (with the shades of grey in the middle).
Hrm...sorta explaining and backtracking and changing the idea of this a lot, 'cause this is actually making me think a bit. Good show anon. Rare form.