The first season had more stand-alone episodes, so I'd point to that to someone who's just getting started. There is the season-long arc of looking for their dad, but that doesn't neccessarily tie in to the cases the boys work on. Once you get past that season, the arcs become more Buffy-like, with fewer stand-alone episodes in favour of eps that tie a larger story together. Me, I think the show is getting stronger and stronger.
I also really like the chemistry between the two brothers. Way more entertaining than Scully and Mulder, who I do like. Perfectly encapsulated by the following exchange, which is not verbatim:
Sam: "Hey, Dean, what I said earlier, I'm sorry . . ."
Dean: (puts up hand to stop him talking) "Please. No chick-flick moments."
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supervij