What are the most and least played classes?

[Vexander]

Both RDM and SAM have a high chance to be very popular during the first few months of 4.0 as players test out those two jobs.

Sword DPS Job has been a high demand since 1.0 so expect a lot of hype, satisfaction, and disappointment on the first few months as people either learn how the Job plays or give up and call it a copy of another job.

Also depends on how well SE balances them. AST and MCH both were -very- undertuned when Heavensward launched, and DRK was fairly niche, though for its specific role as a magic tank, it did well. So far SE has a track record of undertuning new jobs, probably intentionally since its easier to buff than to nerf, where the playerbase is concerned.