You are definitely not the only one to notice how the character of Nick has evolved. And I think George had a lot to do with that. I really think too, despite the lack of continuity for the most part on the show, you can still attribute a lot of Nick's character to different things that have happened to him on the show. And I really think his character's development has been the most realistic of any of them. It just happened so gradually and naturally.
The last time I watched these episodes was probably five years ago, or so, and my thinking has definitely changed a lot when it comes to how I view a show/movie. I mean, even just after the first time Nick has a gun pointed at him (Who Are You, s1) I can see how he's already changing and maturing. I don't know whether the writers or George did this intentionally, or perhaps it just comes naturally with the course of it all, without anyone making a concious decision for it to be so. I think it's pretty amazing though, especially now that it's been so long since the first season aired.
Yep. Sometimes I miss the old Nick tho. But hey...that's what fan fiction and DVDs are for, right?