ZellSF: For Baldur's Gate 1, there's the original version. It's the purist way and plays the way it should. It does however have some very archaic UI design choices that makes it rather tedious to play. Things like ....game doesn't pause in menus.
Grogger: Something I just feel like mentioning as some things are easy to overlook or go under appreciated.
I wouldn't call that archaic GUI design, or archaic as in obsolete or bad. That was a deliberate game mechanic decision to mimic simulating not being able to dig through your backpacks in the middle of combat. Personal preferences aside it's easy to argue that it adds to the game in regards to the simulation / immersion. It's also something that directly affects other game mechanics, such as quick slots and weapon slots.
Without the game un-pausing in the inventory the mechanics of those quick slots become useless, which is to limit what the player has access to during combat without going through the backpacks in real time. In regards to the weapon quick slots, not un-pausing, has an added affect in that only Fighters would get access to more weapon slots so it effectively removes deliberate benefits some characters/classes have.
This is somewhat comparable to not being able to change armour during combat, the same simulation/immersion logic.
Just wanted to point out this isn't some archaic interface design, or obsolete limitation due to not being thought out and matured, quite the contrary.
In this regard the GUI is simply a suitable representation of well thought out game mechanics. Unlike for example being unable to purchase multiple stacks of items which is an example of the GUI being archaic, along with the negative connotations.
Here's my thoughts on this:
To me, quick slots feel more like a user interface shortcut, not a gameplay mechanic meant to limit what a character can do. From this standpoint, it feels like the relative advantage that fighters get due to having more slots is not a deliberate gameplay mechanic, but rather a limitation due to limited screen space, especially when it comes to multi-class characters. It also has one other issue, in that it makes the game more of an action game and less of an RPG. It's also inconsistent iwth the way the game's pause otherwise works, and is the exact opposite of what many players expect (I can cite the Zelda series as one popular series where the game pauses when you go into the menu to switch equipment).
Also, allowing the game to not be paused in the inventory menu allows more opportunity for bugs to exist. I can, in particular, cite the scroll underflow glitch (where you hold the scroll with the mouse while the character is busy casting the spell, which can allow you to end up with negative (or, equivalently, a large number of) scrolls as an example of this sort of bug.
(With that said, there is one advantage to being able to open the menu without pausing the game; in a speedrun (or if you're in a hurry otherwise), you can give your party a move order, then manage your inventory while the move order is in progress; works fine as long as you don't try to traide items, in which case you might run into one of the game's other annoying rules.)
The inability to change armor during combat falls into the category of game mechanics that are intended to be realistic, but make the game less fun to play. One problem here is that, because battles take place on the main screen (instead of on a separate battlefield), the game's notion of whether you're in combat is sometimes inaccurate (this is also an issue when trying to rest or save sometimes). Also, it's possible to unequip a character's armor during battle by killing and reviving that character, and the inability to re-equip the armor is especially obnoxious here (I find that even having the character's inventory dropped on death is enough to make me want to reload instead of continuing after a death, and I feel that's bad game design). I could classify this annoyance in the same category of range limits on inventory trading (having to go out of the inventory, move the offending character (which requires dealing with the game's poor pathfinding, as again the developers didn't include any free non-cheat way to move without relying on it), as well as area transitions that require the entire party to be nearby to activate ("you must gather your party before venturing forth"). This may be realistic, but it does not lead to fun gameplay.