There have been 3 big loopholes in BG:
a) Gold loophole: Current behaviour:
In many shops you can't sell stolen items. In some you can, like in the thieve guild. That is how it should be, after all it's the fence in the thieves guild. But, you can resteal there and resell stuff in the same gulld again over and over. Expected behaviour:
I think stealing in the thieves guild should be very difficult at first, and just unavailable once you are the guildmaster yourself.
b) Spell learning XP loophole: Current behaviour:
While it was important that some patch did introduce the option of deleting spells from the grimoire (so new players don't get overwhelmed with planning the next 20 character levels of spell selection in advance for their below INT 18 human wizard PC, you could get indefinite experience by deleting and relearning the same spell over and over. Expected behaviour:
Somewhere in the character there should be stored, which spell he already had learned, even if deleted later on. For relearning such spells no XP should be generated.
c) Monster XP loopholeCurrent behaviour:
In Saradush you can't target the giant attackers beyond the city wall. But at the right position, the PC AI script can. You just select some ranged combat AI, stand on the right spot, and go away. Every 20 minutes you check whether no character got a catapult hit. Infinite experience ingame loophole. Expected behaviour:
Monsters, that can be targeted only by AI scripts (but not by the player) should give 0 XP, and/or the AI should also not be able to target them.