From top of my head I can recall four horsemen of annoyance and boredom (just list, without ranking):
1) Level scaling
2) Endlessly spawning enemies
3) shitty controls
4) Weapons and armour having level requirements in singleplayer game
Now to explanation:
Level scaling - nuff said. People were shitting on Oblivion for many autisitc reasons, but this one flaw is reason why I wasn't able to feel comfortable at all in this game and tried to mod out it as much as I could. But no matter how you can try to mitigate such disastrous design decision - aftertaste still remains, because game was designed this way.
Endlessly spawning enemies - I can enjoy the game if it has only 1-2 levels/maps with spawning enemies (while clenching my teeth with annoyance), yet my eyes will roll
every time when I have to slog through this part of the game.
I can play and finish the game if levels contain endless respawns (like System Shock series) and recognize good qualities of the game, but (it pains to admit it) I would think twice before replaying such game and my mind will annoy me with questions "from where these bastards even come from?" I also would try to finish such game faster, just to spare myself from prolonged exposure to such "immersive" feature.
"Honorable" mention deserve also respawns in Wizardry 8. People had many problems with speed of combat, meanwhile I quickly became annoyed and infuriated by sheer fucking amount of encounters without option to avoid, which respawn on top of that and wasn't able o bring myself to finish the game.
Shitty controls - if I am not comfortable with controls, I most likely will not play the game, unless I force myself to do this (I do not recommend playing console 3d platformer games on keyboard, this brings pain even after a year past completion, especially rainbow/carpet level in Italian plumber game, knowledgeable people will understand what I am talking about)
Weapons and armor having level requirements in singleplayer game - one of most idiotic things invented in history of mankind, if MMOs using this method as one of many crutches to economy (why nobody invented something better already ffs?) it does NOT means single player games should have this shit too.
At first, I thought adding RTwP combat to the list, but I considered several things:
1)there exist game with RTwP combat that I liked - Planescape: Torment, even though I liked it
despite RTwP combat. And there pre-defined companions.
2) there exist series with RTwP that I cannot bring myself to hate - Icewind Dale series, never finished any game in it, but you can create our own party from start (which warms my long jaded heart) and it feels more like adventure than Bioware's crap, snowy maps is bonus.
3) I heard there exists a game with good RTwP combat (47 Calibre, I think?), never played it though
Still would play any future releases only if there Turn-based mode and mercs, because I can forgive devs of earlier games for trying blindly catch undeserved success of BG games, while new devs should know better than their predecessors.
Also, honorable mention of Might & Magic: World of Xeen, for making me starring blankly into my reflection on black monitor, while comprehending "what I am doing with my life".
Because this game has aging system and visiting shop takes 1 day and training yard takes +1 day per character in party. Look, I know that some people like shopping, but 24 hours is a bit much and it does not bring me joy knowing that every time when I forget something in shop - my characters 1 day close to death. It brings thoughts about my aging and demise, not very bright thought to have while playing game.