However, other than a very decided opinion on the stupidity of Xander's non-marriage to Anya (the writers' fault, not the characters), I haven't spent much time thinking about early Xander or High School Xander.
Watching Season 2, I've come to appreciate all over again how well-written and funny the show is. I've also come to appreciate Xander's character and, naturally, Nicholas Brendon's portrayal of Xander.
Joss Whedon has said somewhere that Xander is basically "Joss in high school," only (quoth Joss, not me) much better looking. Nicholas Brendon isn't really my type (I prefer rugged actors like Robin Sachs of Ethan Rayne's fame). Still, he is cute, and yet, and here is where we get to Nicholas Brendon's awesome acting, he manages to sell the whole I'm-a-geeky-unpopular-kid-who-uses-humor-as-a-defense-mechanism persona.
When you consider how much Seasons 1-3 of Buffy rely on faux high schoolers, the success of those seasons is remarkable. I believe in the teenness of Willow, Buffy, and Xander in a way I never believe in the teenness of Smallville's cast. Nicholas Brendon's acting is part of the reason. He captured the essence of 16-year-old guy; he used whatever background/memory/experience/observation he had to give us the mannerisms and emotional responses of a male teenager.
To return to Xander the character, the success of Xander the character rests, I believe, on Xander's humanness. Xander is fundamentally good, but he isn't heroic-rush-to-the-rescue-and-look-soulful good. He's just average guy good, real life good. Even in "Inca Mummy Girl" where he gets to play the romantic hero, he does it in a very human, 16-year-old boy way. He shares Ho Hos! He tells silly jokes! He takes Inca Mummy Girl to a dance!
Xander is the guy who is brave in spite of being freaked. He is the guy who does the right thing eventually. One of the most mature/human things Xander ever says is after he returns from his hyena/pack phase. To Giles he says: "Shoot me. Stuff me. Mount me." Yeah, he was being a jerk. It's over. He'd rather not remember. And he's never going there again.
Xander's one flaw is a tendency to hector. But again, this tendency makes him human. It isn't so over the top that you start to detest him; it isn't so understated to make Xander too good to be true. I hold Xander more responsible than Willow for the whole Xander-Willow fiasco (Season 3). Yes, yes, I know that in general terms, they are equally to blame, but Xander has a tendency to take a situation and run with it. It's a type of me-me reaction that accompanies hectoring. It's, well, it's so 16-year-old guy.
And yet, this is also the Xander who buys Cordelia's dress without telling anyone. He always protects Willow (watch the show carefully to see how often Xander puts Willow before everyone else). Also, as I've stated elsewhere, I believe Xander is the only one who really understands how lonely Buffy really is (here Xander stands in for Joss).
Lastly, Xander is just funny; Nicholas Brendon has excellent comedic timing. In my favorite episode of Season 2 "I Only Have Eyes For You," Willow makes scapulars for everyone. Xander responds by saying, "And what are we going to do when we find the spirit, Will? Flip it?"
Ohmigosh, I'm laughing so hard, I can barely write.
Okay, so maybe you have to be an English teacher to think the transposition of the words scapular and spatula is just hilarious, but my point is, the joke works to a large extent because Nicholas Brendon makes it work. It's one of those word jokes that are easily lost until you've watched a movie a couple of dozen times (like the "Moby Dick" joke in Finding Nemo--yep, really, it's there). There's lots of those jokes in Buffy, and the ability of Whedon's cast to deliver said jokes deadpan is a huge part, I believe, of Buffy's success.
So kudos once again to the first three seasons of Buffy and extra kudos to Nicholas Brendon's Xander.