I don't like multiclassing.
I am ok with the PHB Magic Initiate because it allows non-casters to get a little spells and maybe open up new tactics (if anything, it's a bit weak as it's only a single 1st level spell).
I am more ambivalent on Martial Adept. I think it's important for Battlemasters to have an option for getting more maneuvers, since the default amount is scarce. I don't like diluting the charm of the subclass by making its defining feature available to others, but the single superiority dice granted by the feat is such that you'll never outshine a real Battlemaster even remotely (in fact they could've called it "inferiority dice").
But more multiclassing feats I really don't want. I hate to see that there are 2 well-known classes in the game which many people believe they got the shaft when designed, and instead of beefing them up with options to make them more attractive, they make then even less attractive by taking what good they have and give it to everyone else in the form of feats. I really hope they won't give out Sneak Attack or Rage as feats otherwise it would be the final nail in the coffin to class archetypes. I say, go play Gurps if you want a classless game, at least you get the real deal.
Giving out Fighting Styles as feats is however the worst offender from a balance point of view rather than concept. A Fighting Style is typically really powerful because it gets used potentially every single round in every single combat. Once you choose yours, you're supposed to stick with that weapon configuration as much as possible, that's why a second Style is significantly less valuable than the first. But Styles are one of the most important things to make martial classes still be better in combat than others, especially Fighters.