I know what you mean about not being in fandom in the same way; I adored BSG, and it was a pretty ideal setup for a first fandom. (Lots of women in important roles seems to be a huge factor in that.) I found my way online in early season three, and there were two year-long hiatuses after that, so it was a long stretch and we had to find ways to keep up our fannish interests.

I haven't experienced anything quite like it since then. Person of Interest was my revelatory fandom: shows focusing on two white dudes attract a fannish crowd that I don't enjoy as much. And then I didn't watch The Man from U.N.C.L.E. until almost three years after its theatrical release.

(I don't know why the whole hyperfixation thing happens with some media and not others. I liked Doctor Who, and even wrote a bit of fanfic for it, but it didn't ever grab me the same way. And The Witcher is AWESOME, but I can't really think of anything I want from fandom... I just want more episodes.)