As someone who only played Baldur's Gate 2 when it came out and enjoyed it greatly, I don't think I would've gotten there if I had started with the first game. It has not aged gracefully.
The first problem that drove me away from the game for a few months was the ridiculous difficulty. I enjoy a challenge, but this game is just unfair. Almost everything in the game is based on random chance, and the odds keep stacking up against the player. For hours and hours I would have to reload almost every fight 5 times, because my main character would get one-shotted. This ends the game, even if someone in your group had the skills to resurrect her. But more than 30 hours into the game, I'm not even close to getting such a spell. There are temples that offer such a service, but having party members die gets expensive FAST.
The second problem is the outdated game design. Mages are next to useless in the beginning of the game. They can cast one or two spells per day, after which they must rest. Resting in the wilderness risks an ambush, so it is not something to be done at a whim. Resting at an inn is safe, but travelling to one also risks an ambush. The encounters are not usually hard, but they are annoying and can sometimes lead to casualties, especially when the party is weakened. Therefore, while mages and clerics are immensely useful later in the game, they're basically just dead weight for the longest time. A cleric is somewhat more helpful than a mage, though, because they can wear armor, swing a weapon, and have access to a wider variety of spells right out of the box. You will not get a real cleric until fairly late into the second chapter, though, unless you created your main character a cleric.
Also, did I mention enemies spawning in areas that have been covered by fog of ware for mere 15 seconds?
And this leads into the third problem. Baldur's Gate aims for a real D&D roleplaying experience. While I am the first one to applaud any game that tries such, it must not interfere with the gameplay. BG does so in spades. Firstly, aside from your main character, every group member has a personality. But this is good, right? Well, yes and no. Some personalities are simply less tolerable, ranging from simple whining to almost comical villainy. This becomes a problem, with the reputation system. You gain some when you're acting like a decent human being, and down when you're being a ♥♥♥♥. Good characters like the former, evil characters prefer the latter. The choices are usually either or, and while there is often an indifferent option, it usually passes an opportinity to gain experience and powerful loot. Needless to say, going too far into one extreme will cause characters of an opposing alignment to get emo and leave. To make matters worse, most NPCs come in pairs. If you choose to ditch one, their companion will also leave. While none of them are completely useful, there are some that are much more useful than others.
Pathing is also abhorrent. If a character's path is blocked, even temporarily, they will make a round trip around the map if they have to, happily running into enemies. In combat, however, they have a tendency to get stuck on each other trying to get close enough to attack.
Itemization is also rather capricious, but before moving into the essentiae of it, I'll mention monks. They're a class that cannot use armor, and should generally not use weapons. While this causes them not to require the most common items, they become even more dependent on rings, amulets and such that are very rare. Other classes also have their own problems. Using any weapon without proficiency will incur penalties for using said weapon. Most classes gain proficiencies very, very slowly, so choosing them wisely is important. But again, the game pulls a fast one; while enemies drop more weapons than you can carry, it is mostly short swords, halberds and short bows. Other types are really uncommon. This is important, because for reasons pertaining to the story, metal weapons can break. Permanently. This is, again, dictated by random chance, but I've had around 10 weapons break thus far. It gets really annoying when there is only a steady supply of weapons that no one is proficient with. Apparently magical weapons are not affected, so while the problem adds to the frustration of the harsh beginning, the selection of weapon types is really subject to the designers' whims. It's pretty much impossible to predict which weapon proficiencies will be useful in endgame.
The game's only saving grace comes from the world and the story. I haven't been able to uncover much of it, and while it does seem interesting, I don't find the game fun to play anymore.