To answer your question, Exodus, a lot of things. The atmosphere is important; this can include the lighting, music, sets, props, etc. The acting is even more important because it's hard to be engaged in the story when the characters aren't interesting and don't act like real people. The story is just as important as the characters. When it comes to Trek (and most film and television), stories that make good use of the characters and develop them are generally my favorite. Finally, and less importantly, is the script. The reason this is less important to me is because a great actor can salvage even the worst script, which is what happens all the time on Voyager.
So, about Before And After, what was it missing in my eyes? Well, for starters I'm just not a big Kes fan. I don't dislike her, but at the same time I don't think she's one of the most interesting characters on the show (although I must say I like her longer hair
). With that said, her actor gives a fine performance, so it would be unfair for me to hold that against the episode.
I have mixed feelings about the story. On one hand I loved the bits about The Year of Hell; seeing Janeway and B'Ellana killed shocked me and really threw me for a loop. Seeing Voyager torn apart and the crew barely holding together was definitely moving.
One thing that bothered me from the very beginning was that I knew none of this was actually happening. When Chakotay entered the scene and was addressed as "Captain" I instantly knew that this was just "another one of those episodes where something strange has happened to the timeline and it'll all get fixed and be back to normal by the end. Snooze."
I hope my score makes a little more sense to you now.
Note that my score is now 5/10.