I'm thinking that it's difficult to maintain a two super powered (i.e. slayer / vampire, two superhero) relationship, which is one of the reasons Superman and Wonder Woman never worked out (in the past and hopefully in the future)... tooooo much power together in a relationship makes for one combustable relationship.
Lois, being non-super powered, brings another aspect to the relationship. It doesn't mean that she's the less-equal member of the duo, since, in my opinion, Clark brought more love. Also, it reminds Clark that he has to be more careful with her and he has an place to go (other than to his folks) where he can get away from being a superhero.
I agree with Michael. Lois and Clark were in a different stage in their lives, a stage where the testing phase of relationship is basically through and the settling down phase had begun.
Buffy was in high school and college and nowadays that's a bit early for settling down. I believe that she could have worked things out with Riley if he hadn't bailed so quickly after she dumped him and they had had time to reconcile.
I always liked Spike and hoped they could make it work; besotted Spike is the reason I kept watching Buffy.
True, that relationship was disfunction on it's ear, but Spike was willing to get his soul back to win her love, which was more than she was willing to do for him. I was actually working on fanfic story (that summer before he got his soul back coincidently) which had Spike summon his soul and it came to Sunnyvale (dale?) in its new reincarnated body (as a missionary of all things).
I wanted to clobber Xander after what he did to Anja, especially since it was HIS idea to get married in the first place.
From what I've watched of Smallville (first 4 seasons) the Buffy / Angel relationship (super powers aside) is very similar to the Clark / Lana relationship. They are soulmates that are cursed never to be together. Either that or it's the whole high school drama thing again.