How does not allowing multiclassing and/or feats impact the game?

[Mjolnirbear]

I firmly believe there isn't a character concept that can't be handled by the basic classes. We did this for years in earlier editions, just because 3rd eds prestige classes and rampant multiclassing made it easier doesn't mean we lost the ability to do it.

Whereas I disagree. You can approximate many, but then I say "gish".

And you say EK.

I say EK is a fighter with a couple magic tricks. You say bladesinger. I say I hate elves, and also a bladesinger is a wizard with a couple melee tricks. You say paladin, I say 'too much nobility, also not arcane'. And you, as a reasonable DM, maybe say you'd be willing to reflavour a paladin to something arcane, switch out spell lists... But while generous, that requires a DM buy-in, which à) only works at your table and b) is less likely to be granted by a DM that won't even let you multiclass and c) I want cantrips too.

None of your hypothetical answers were wrong even. Those are all great ways to make a gish. But you can't cover every option.

I've never played a ninja. You immediately mentioned shadow monk, which is, again, an excellent ninja. But while the teleport is admittedly cool, I want mine slightly armoured and be a backstabber and expert stealther. But he needs to be able to do weird things like cast magical darkness, and see in magical darkness. Only one class can do that.

I want to play a lycanthrope. Obviously I can't play the monster manual werewolf, not most games and not one that doesn't allow feats (edit: to clarify, if feats are unbalanced, then so darn well is MM lycanthropy). But I can build one. Two levels of druid and one of barbarian, and I have a bear claws, a savage bite, and resistance to a lot of damage.

And there are things that can come up mid-game.

How do you do an experienced fighter, who then makes a bargain with a tricksy Fey? How about a paladin who takes up a life of prayer and healing and takes a vow to touch no weapon? Perhaps after six levels of wizard, you realize you're still too squishy and feeling a little sickly, so you put down the book for a while, and go out to get some sun and broaden your horizons and learn to take a punch.

Both the gish and the lycanthrope concepts are real, not contrived, thought processes I've had. I really hate elves. I am extremely dissatisfied with EK. And I haven't got to play a lycanthrope yet but I can legally (and fairly) approximate it if only I can multiclass.

Someone will say I have a special snowflake syndrome. Whether I do or not, the fact remains that allowing feats (and multiclass) can both help you pin down a concept more precisely than if neither were and option. I'm playing the game to have fun; if the best way (to me) to play my character is blocked by a big Wall Of Nope when I can clearly see a perfect solution on the side where more options exist, the game will be less fun, and the restriction unreasonable to me.