I had only seen bits and pieces of Supernatural and never cared for it in the past, but starting around Season Four I finally started to enjoy it, and by the time S5-6 came around I truly liked the show and looked forward to new episodes.
So I've gone back and watched Season One... and it's pretty crappy...
The trademark humor that I love so much about the show is just not there. Also, Dean doesn't even get his Batman voice until towards the end of the season! And there's basically no storyline whatsoever, just a handful of "arc" episodes.
And after hearing about the Dad so much for all those later seasons, he's pretty disappointing. I like the actor, but he's just kinda there, he doesn't really have a personality. Bobby is a much better mentor character.
Do things get better in Season Two?
When does Castiel first appear? He's my favorite character