Sorry for cross-posting.

In previous games, to cast two magic missiles the mage would dedicate one spell slot to each when the spells are prepared. Same thing with cure light wounds or whatever; one casting, one spell slot. I haven't been able to do that with Baldur's Gate 3. As a wizard or cleric I can only prepare a given spell once and (except for cantrips) cast it once per battle.

Is this the way it is in this game, or have I missed something? In one way it is refreshing to be forced to use other spells... but it feels limiting and makes wizards substantially weaker.

It's how 5e works. You will get additional and bigger spell slots overtime, increasing the amount of spells you can cast.

Right... but even with additional slots, I can still only cast a given spell (such as magic missile) once, then?

It's actually much more flexible than previous DnD editions. You can cast the same spell multiple times, as long as you have available spell slots of the spell's level or higher but you only have to prepare each spell once. Concrete example using a screenshot from fextralife: https://fextralife.com/wp-content/uploads/2023/08/Baldurs-Gate-3-Controller-UI-1536x864.jpg Note that screenshot is for the controller UI and is different to the mouse and keyboard UI but it's what I'm familiar with, as a controller user.

In that screenshot, Shadowheart is a level 2 Cleric and has the Cure wounds spell prepared (the blue cross in the left most radial dial). She has a maximum of 3 1st level spell slots shown at the bottom of the screen. She has used 2 of these and only has one available slot left, the blue one. With all 3 available, she could have cast Cure wounds 3 times. As you level up, you get more spell slots and at higher levels. The Cleric progression is according to the table at https://roll20.net/compendium/dnd5e/Cleric#toc_3

You can cast a spell using a higher level spell slot than the spell. For example, a level 3 cleric has a maximum of 4 1st level spell slots and 2 2nd level spell slots. Cure wounds, despite being a level 1 spell, can be cast with a 2nd level spell slot. This is useful in a couple of situations: if you've run out of 1st level spell slots or if the spell scales with spell slot. For example, I believe Cure wounds will cure more hit points if cast using a higher level spell slot.

Yeah, from what I can tell Preparation casters work slightly differently in 5E than they did in earlier editions. As twistedpony said, Preparation casters like Wizards and Clerics typically have a massive spell list, but they still need to choose which spells they want to currently bring with them (i.e. "preparing" them, although it may help to think of them like a shortlist.) HOWEVER, unlike in 3rd or 2nd Ed, once you have your shortlist, you can freely cast any of your shortlisted spells using your available spell slots, similar to a Spontaneous caster. So if you were a Level 1 Wizard with 2 level 1 spell slots, and you've chosen Mage Armor, Magic Missile and Sleep as your shortlisted/prepared spells, you could cast 1 Mage Armor and 1 Magic Missile, 1 Magic Missile and 1 Sleep, or 2 Magic Missiles as the needs of the adventure dictated.

Spontaneous spellcasters don't need to make a shortlist; their entire library of known spells IS their shortlist, but their limitation is that their spell library is a LOT smaller than Preparation spellcasters. If your spellcaster's role is very focused (for example, if your Sorcerer solely exists to chuck high damage AoE spells around), a Spontaneous caster will likely suit your playstyle better, but if you want a caster that basically has a spell solution for anything you might encounter, a Preparation caster might be more up your alley.

Note that, again unlike earlier editions (and this might be a BG3-specific thing, I dunno), you can freely change your shortlisted spells any time you're out of combat (although you won't regain your expended spell slots until you long rest). This means that my Wizard could wake up in the morning, cast some Divination spells to see what the day will bring, then swap to his regular combat spell array. Then, in the evening, he arrives at a town, swaps out the combat array spells and re-slots more interaction-focused spells like Comprehend Languages and Charm Person for the remaining spell slots he has for the day.

This is BG3 specific. In DnD5e, you can only change prepared spells after a long rest. Though I'm very glad they changed how this works for BG3. Being able to scout what enemies are ahead, then prepare specific spells for the encounter is much more fun than trying to rely on a list you generically prepared at a long rest. It allows for much more creative solutions in encounters. Or worse, the DnD5e restriction would encourage players to long rest after each and every encounter, making the mechanic pointlessly annoying rather than simulating an adventuring party pushing themselves to the limit during the day and enjoying a well earned rest.

You get a certain number of spell casts for different spell levels, which will increase as you level up. So you might have, for example a 3 uses of level 1 spell, 2 uses of level 2 spell and 1 use of level 3 spell. But as you level up you get more spell slots and you will be able to cast spells more times or more spells. There are also potions in BG3 that give you back a spell slot, so you can drink it and cast again.
There are also some skills/spells that you can indeed cast once per short rest or once per long rest, but generally you get a certain number of usages per spell level, not the spell itself. Also, you can cast a lower level spell with a higher spell slot, which will increase the power of a spell, but will use a higher level spell slot. An example of this can be heal, which will heal a lot more if you use a higher spell slot for casting a basic level 1 heal.

Also, this is mostly wizards. Other classes will have somewhat different rules, like Warlocks. They have a specific number of spell usages and that's it.