It's not really about age though. Have you seen the women who watch soap operas? They aren't all spring chickens who should know better. LOL. Now those are worth banging your head against a brick wall from. Roswell, for all its teen angst, has a plotline with that teen angst/forbidden romance hook.
I bet there were a lot of bored housewives tuning in to shows like Buffy, Angel, Roswell, etc... And I bet you a lot of them were into the romantic plots. Those shows also attracted nerds in droves.
Funny thing, but the target Dawson's Creek audiences fled the shows when they got too fantastical or involved. It happened to all three of those shows. Well, Angel never even had a shot at the "Gilmore Girls" audience.
As for the Jossverse, the driving force of that fandom is STILL Bangel vs. Spuffy. Riling up two of the most infamous 'shipper groups is always a recipe for epic forum battles. The fact that those two couples are left hanging in the air after 12 years of storytelling (17, if you include the movie) is practically the second life of the franchise. On the other hand, Roswell was kind of wrapped up in a nice pretty bow. Though I would love to see the characters face being on the run. Roswell comics? I wish. Alas, the fandom is definitely in hospice at this point.
While the Jossverse has Angel and Spike as the most prominent female hooks, in the Roswell world, it's Max and Michael. Thankfully, in the latter, they never competed for the same lead.
And for the target audience, the relationships of Max/Liz and Michael/Maria WERE THE POINT.
The plot was mostly there to throw those relationships for a few wacky loops. You know, like, Tess (whose entire purpose is to threaten Max/Liz and to contrast the way that Max, Isabel and Michael grew up as humans). It's about those relationships with a side of discovering one's identity (something teenagers/young adults relate to)--the existential aspect to the show.
The sci-fi was there as flavor and to add a mystery that unfolded (again, the existentialism).
And yeah, I love those Christmas episodes. It's a tough race between Max and the cancer kids (yeah, I'm a sucker; it made me cry) and Santa-Michael with elf-Maria (hilarious).
Summer of '47 (it's just made of awesome--a flashback where the main characters act out a past they weren't in), A Roswell Christmas Carol and The Departure are probably my favorite episodes. All season 2.