Not exactly sure what you mean by 'serious', but I'm going to assume you just mean someone who loves the hobby.
There are certain things I've read/done/bought over the years that I would consider an important part of having comics in my life. (probably too many to remember)
First of all, I'm glad I got to know the owner (and employees) of the LCS I frequented as I was growing up. He was actually the first adult friend I had when I was a kid (back in the early70's). Back then you got a lot of eye rolls when people found out you collected comics. It was nice to be around other 'nerds' and feel comfortable.
I'm also very happy whenever I see threads here (or on other boards) concerning the best written comic stories. I don't care who you are, you can't know of every book out there - especially when the majority of your life was before there were computers. I think, and this is just my opinion, that the younger you are, the more emphasis you put on the art. Whereas, the older you get, the writing becomes more important. I've found/read so many great stories as a result of those kind of threads.
I've also found it immensely informative to read the many books out there dealing with the history of comics. (All topics: publishers, artists, covers art, etc) I just finished rereading this. (I have this in the bathroom )
Even though CGC, et al. isn't essential, I'm glad that I know and understand about them. Without these forums, I'd be scratching my head whenever I saw a graded comic.
One last thing (for now) - re: those Gerbers - why wasn't there ever a DC set put out. I'm not really a DC guy, but it'd be nice to know what's out there. Having the 2 sets that were published has definitely impacted my comic collecting.
Mike
ps - We've all heard it a thousand times, but the most essential part of this hobby is to collect what you like.