I like the fact each character in Dawn Chorus is unique and has backstory, so I'd say that's a pretty important element for me.
There's also the fact most routes (safe for Torulf's) are looking to be more on the romantic/fluff side than sex-related.
(hope you don't mind reading~)
That's definitely a criteria that is important, unless unique means just different and not different from most/all other characters or love interests. Therefore Tyson was/is my favorite love interest of any FVN, who is basically best friend friend, not literal dad and lover all at once for the MC. Backstory isn't really important to me, because characters have to be more than 1/2 dimensional from the start to keep me invested in the first place or the story is intense. You could say a backstory adds depth, but I think it can also be cheating.
The best gay novel trilogy I ever read has the greatest characters of any story I know. Everyone is unique and interacts in a believable way. Their backstories are very intense.
I don't mind a well written sex scene, but it's hard to get them I feel like and can even ruin a FVN (creator too) in my case with Tennis Ace. Romance is always welcome, if it is introduced correctly and executed in a natural pace. Some FVNs such as "cute'n fluffy" (forgot the name) completely fail or ridicule this crucial aspect.