Someone mentioned in that thread the Londo/G'Kar dynamic, and I agree, in a lot of ways, it's the heart of the show. The way their grudging friendship evolved over time was so well done -- you saw how their preconceived notions of each others species wore down as they actually got to know the other person. We saw an oppressor showing remorse and the oppressed showing forgiveness. It's a real testament to the performances of Peter Jurasik and Andreas Katsoulis that these two characters had as much soul as they did.
And Lyta being revealed as the Psi-Corps sleeper? One hell of a "Holy shit!" moment. Her screaming "The Corps is mother! The Corps is father!" still sticks in my head, it was such an intense moment.
I know the show has its detractors, and some of the criticism is deserved -- Season 1 stumbles around like a blind man sometimes, and Season 5 really suffered from the half-assed way it came into existence -- but those middle three seasons are as fine bit of televised sci-fi as has ever been broadcast.