I've been re-watching the third season of Buffy, my personal favourite, and I've hit the second disc and the most wretched romantic plot of the show. Considering we just dispensed with Scott Hope, that's saying something.
Oh, I hate Willow and Xander's affair. It is, admittedly, a mercifully brief storyline but excruciating while it lasts. It never really has consequences, either. Willow and Oz have about one conversation about it total after which its never mentioned again, not that that relationship has long to play out. Xander and Cordelia do break up and have a pretty satisfying reconciliation in The Prom but Xander never really considers why he did it.
Neither does Willow. For a writer and showrunner obsessed with the emotional consequences of his characters' actions, this does seem like a pretty big unaddressed gap. It doesn't tie in with any of Xander's very real emotional difficulties, it doesn't shed any light on the development of Willow's sexual identity (which, with The Wish and bisexual vampire Willow just around the corner, was immediate) and aside from a snide comment by Anya way down the line its never mentioned again.
And, honestly... I just wish it could have stayed platonic all the way through. I'm a sucker for men and women as best friends and, this aside, Xander and Willow are the ideal of that relationship. Honestly, the fact that Xander gets over his crush on Buffy and Willow gets over her crush on Xander and they are friends who are friends and no one's planning and plotting to make it more... I wish there were more stories like that.