At the risk of being branded a Grolim follower of Kal Torak, count me as one of those who think the Spell versatility feature is a bit too...much. The classes that benefit from it are supposed to have other features that make up for the lack of versatility in spell selection. I would much prefer it being an official 'swap spells on level up', or--if the classes in question are perceived as lacking--perhaps give them more unique features like additional metamagics or Invocations.
Then again, I always thought that the Ranger should have followed the Paladin/Cleric/Druid model of spell casting, so there's that. And speaking of those classes, their unique spell casting model is largely due to tradition, and the fact that traditionally, 'divine' spells tended to pack less general punch and versatility than 'arcane' spells (though that differentiation has admittedly diminished over the years and editions). There is also the fact that, also traditionally, such classes had to devote a significant portion of their spell load out to keeping the party on their feet and free of nasty conditions.
I guess at the end of the day, it is not so much a balance issue, but the general design homogenization of the spell casting classes, something that 5e has already taken a step or two towards, that kind of rankles a bit.
dnd.wizards.com
