Imo nothing, even my favorite games can never be a true 10/10 in my eyes.
The biggest reason being my standards for game balance, even in single player games to some extent. I don't like being "broken" unless I've somehow reached such a skill level (be it execution or planning) to deserve breaking the game to whatever extent the game allows. Most games I play never do that it usually has some dumb combo, skill, class, equipment, literally anything that the developers either ignored how broken it was or felt was fine to exist. That can take me out of a "wow this game is amazing!" phase pretty quick, but not enough for me to hate the game itself just more of a criticism that affects my whole experience enough that I can't see a video game as "perfect 10/10" worthy.
I like fairly complicated bordering on convoluted combat systems with a lot of moving parts and/or options, like Xenoblade 2 or Divinity Original Sin 2 for somewhat recent examples, yet I also like reasonable levels of balance so all those moving parts feel equally relevant to some extent. I've just come to accept is a gaming development nightmare that will probably never happen especially without constant balance patches like in multiplayer games. Their will always be some broken combo that wins everything, though I think you can keep it from curb stomping the entire game into the floor by just pressing 2 buttons sort of bad which is how I'd determine if a game is say an 8 or a 6 on a true 1-10 scale where 5 is an average.
Just on a gameplay level I am too willing to criticize or notice flaws in a game for the types of genres I like, so I can never feel perfectly satisfied and I'm too autistic to just say "close enough, 10/10". I can still have great experiences and can recommend games I like, but the magic fades eventually and I just start nitpicking way too much for my own good.
