I am really curious how this is going to work out.
I've played on private servers where everyone is dedicating their time to the vanilla experience.
In Classic there will be a far greater spread of player level and gear progression as many people see it as a side project next to their "standard work" in retail. This can have advantages, there will be probably always groups for lower level dungeons around..and also the lvl 60 dungeon scene will be more crowded I guess.
I do wonder how this will affect PvP ranking, there will be more players doing "some" PvP (as they need to also do their weekly stuff on retail), making the climb for dedicated PvP players in classic easier as there are more players participating?
Well, actually I think people that see classic as a side-project will most likely never see lvl 60...and even if they do so they will find out that dedicated classic players will outgear them by a mile and lose the "fun" in the game quickly. But, everyone should do what they like. If you like to level in classic and just do it for the experience, reach a max of lvl 42, no problem!