For me I don't even see it as retro bikes but " normal " bikes; free of the marketing horsesh1t that adds worthless cogs, over-braking and Range Rover suspension to something as beautifully simple as a rigid steeler ( maybe a nice Marzocchi fork for summer or man-made trails ).
for me it has to be the variety on here. I was always used to modern bikes, disc brakes, full suspension and all that. But after getting an old marin project and joining on here I've been converted. And the fact that there's no badge snobbery like some groups. It doesnt matter if you ride a £10 apollo or a £1200 racer. You always feel just as welcome.
oh and ummm lots of pictures of jaw dropping bikes helps as well