This is very curious, because two games which I played so much that I know them like the back of my hand are some of Might and Magic games (specially II and World of Xeen) and Shining Force II but this huge knowledge of them never bothered me or diminished my enjoyment out of them even with multiple replays.
That said, yeah, I had the same reaction with Baldur´s Gate, specially the second game, I loved them much as anybody else, but when I tried play them again, I couldn´t do it - I managed to finish again the first game, but I give up on the second game, which I found myself not enjoying it, the breaking point was that side quest with the dungeon with the small beholders (the one which they keep casting a paralyze spell over and over and you need it to dispel it over and over again).
I still could not put a finger on the why this happened this time - I know the easiest thing is just blame the rule system, and to be fair, by time Baldur´s Gate as around, AD&D 2 rule version was as it end begin over stretched with optional rules and early 3 edition rules add on. But, on the other hand, I played the Pool of Radiance and Dark Sun games form SSI or even later games such as Birthright, which also use AD&D rules in very literal version (in some cases not even the 2 edition rules, but the first edition), and this issue never happened.
My only guess is that Baldur´s Gate, specially the second game, might require huge amounts of pre-planning even before you play, due things like your main class* mostly like de decided depend of party composition (which require you to know them before hand), which also mean you need to keep in mind the alignment system and possible party member incompatibility and due the second game higher level and more difficult encounters mean you need a much more efficient party, which result, again you at least aware of certain parts. Now if you are, or once you are, aware of all of this is maybe possible to play in a such optimal way, but also mean you know things you can´t to or mostly like not going to see.
LaserJesus just commented on mages and yeah, specially in BG II, they can become really annoying, my memories are of every mage having all protection spell up, which mean that you need to memorize tons of dispel spell of different kind every time.
A major difference I think exist between older SSI AD&D games and BG, is that first ones, where almost too happy to hand you experience or rewards, while some games did have a maximum level, you mostly like to reach before middle of the game or before, same as rewards, this often get a bit out of control, in Dark Sun you might be swimming in magic weapons per example, while the Pool of Radiance games had often to come with a excuse every game for you party losing all stuff they had. But this meant that was easier to blast the game. BG and other games after I think are a lot modeled on the idea of “handing XP or magic stuff is bad” so they had this much slower pace, which might result in a much slower pace for the game itself.
- Don´t forget you need to know which weapons are more available before spending weapon proficiency skill points or begin stuck with a weapon which almost don´t exist in game. This is something I kind became aware listing to the System Mastery podcast, where I think once they comment on how this rule made fighters kind very limited. Older CRPGs based in AD&D never implement this rule, so you never need to worry if you going to find the weapon which you did specialize or not.