1. Story (Five Seasons, One Arc)
Creator (and writer of 92 of the show's 110 episodes) J. Michael Straczynski - hereafter referred to as JMS - decided he wanted to tell a story the likes of which had never been told on television before. Babylon 5 is a show with a dense backstory - alien civilizations, distant wars, dozens of regular and recurring characters - that makes for a rich, epic world. JMS planned out all five seasons of the show in advance, intending the story to unfold like a novel for television. Each season functioned as a single act, each episode moving the story forward bit by bit.
2. Character Growth (Everyone Changes)
A lot of shows have what I call passive character development - a lot of things happen to the character, but the character himself remains the same. The way he reacts to Event A in the first season is the same way he reacts in the last. Not so on Babylon 5. Characters change and evolve; some become better people, some do not; the idealistic become jaded; the weak become strong; the bitter become wise.
3. Speeches (Sci-Fi Requires Great Speeches)
Is there anyone whose heart doesn't beat a little faster when someone makes a grand, courageous, death-defying, truly awesome speech? Most of us live ho-hum lives, and don't generally have the opportunities to really tell someone to fuck off in such wonderful, grandiose ways - watching TV characters do so is living vicariously, and few shows have as many or as great speeches as Babylon 5.
4. Bruce Boxleitner (I Mean Seriously Just Look At Him)
The dude is handsome.