I'm pretty sure sooner or later, world transfer will allow legacy players to move to new server or the opposite.
It's just for now, they wanted the new server to be untouch and new players to have a new experience and not interfere by legacy players.

Old server aka legacy server issue.

It's bound to happen in legacy server, those with a max level would run around aoeing some low level mob to accomplish their quest or farm mats. Or it'll take a hit or 2 to fully wipe out low lv mob.

Legacy players who now had max their combat jobs will concentrate on farming mats to skills up their tradecraft.*For those who had always combat first without skilling their craft*

Those who have max their craft and combat job will be controlling the market. They could farm mats as easily as they want. They no longer need to skills up on their craft anymore. They'll probably run around digging for mats and then put it up for sale. Since there's new players flocking in, they could fetch a hefty price sum for those heavy sorted mats needed for skilling up crafts.

RMT rodents will continue to exist with their past account and manipulate the economy.

New server issue.

Players will all be skilling up from ground level. Whatever you try to farm or level upon on the field is bound to be shared by various players.

Mats wont be freely available for the moment if you are a heavy craftsman. The price will be expensive.

Though new server, players who have been playing since 1.0 will join the server. Their character wouldn't be able to do so. But they already know the direction and where to start. It's just a matter of time, They have whatever they want in a short time frame.

RMT will join this new server as new player. Players could spot them easily by their weird action,gibberish name and gibberish engrish.
Yes, they'll be moving in large scales across various places.