I don't think the game designers intended for anyone to complete the game 100%. There are lots of things in this game an individual player might never be able to find on his own. This includes quests as well as hidden items. But thanks to the magic of collaboration, it's possible not to miss anything if you seek outside help.
For example, there is a field on the western side of Nashkel where a suit of armor and a pearl are hidden. There is a locked house on the eastern side where you can find the wife of a miner named Joseph. How many people would think about breaking in? If you found Joseph's ring inside the Nashkel mine, you can return it to her for some experience. Inside one of the taverns, you can meet Volo, who is a well-known fictional character like his friend, Elminster. (You might have seen their names in the manual.)
In the Nashkel Circus area, you were probably approached by a man who offered to sell you a Stone to Flesh scroll for five times the normal price. Nearby was a petrified woman you could have rescued. (It's Branwen, one of the NPCs who can join your party.) Also, did you meet Xan? I can see how a player might miss him.
There is also an, uh, oddity in the Nashkel cemetery, but it's something you will probably be glad you missed. If you click on one of the headstones several times, a ghost (for lack of a better word) and his minions will appear and they will in all likelihood kill you. Good luck trying to defeat them.
There is an area to the southeast of Nashkel where you can meet a man who has seen you in his dreams and a young man who is on the run from the law. Hidden inside the wall of a cliff are a powerful ring and some other treasure.
Remember meeting a strange old man shortly after you left Candlekeep in an area we call the Crossroads? There's a Ring of Protection +1 hidden on top of a rock just to the north.
Beregost has a couple of quests you might have missed. There's a woman inside a house who is waiting for some news about her husband. You might have met the ogrillons who killed the messenger who was carrying a letter for her. If you have the letter, you can deliver it to her. In another house, there's a man you might have met in Candlekeep (he asked you to retrieve a scroll from a monk) and he will offer you a quest. On the upper floor of an inn, there's a rude man who has a special item that can be pickpocketed.
To the south of Beregost, there is a house that is partially underwater. You can meet the owner of the house later in the game, but you might not realize it unless you do something that might be considered evil--that is, demanding a reward after you have rescued someone who is obviously unable to pay.
There are also some caves in several areas, and some of them are easy to miss. For example, there are three caves on the southern side of the Gnoll Stronghold. The first time I played the game, I only found two.