for me it's "no apparent bugs" and also "I know it when I play it."
I think when you own a game from beta like I did with XMLE and STLE that you can see the bugs in total from jump and see as some are fixed and some are not, I see the code from when it was poor and incomplete and see it as it get updated. But those games still had problems (to me). ST had bugs, XM has something not quite great in the whole code (it didn't feel cohesive).
After that I could not keep those two pins in the face of better (to me) coded games like Tspp, Woz, SM, TWD etc.
Not saying ST or XM are bad but just imo life's too short to put up with some things you don't like and when you can get something that shoots just as good with better code.
For example STLE has a ball save kick back and also a behind the flipper shot (a very important shot). On both there was a chance I could pass the switch but still pass the kick back and drain. The code should be programmed to know and give me a virtual ball save. I've seen it on other pins it's not complicated to figure that the kick back missed. But for whatever reason Stern would not fix it. Also sometimes the behind the flipper shots would not register and I would have made a very important shot and it would not recognize it. Now the ball gets into the shooter lane and the only way for this to happen is to make the behind the flipper shot yet the code does not recognize this and give credit for the shot and so (to me) thats a bug.
I am not saying SM or Tspp are innocent of bugs but when I play them, they are not apparent to me and so they don't bug me.
Another thing to me is when a code fits together it just feels right. Some games maybe XM and ST stand out, they just feel sometimes like a double handful of modes that are thrown into the code but maybe the overall structure doesn't make them feel authentic in the way that for example LotR with just as many modes but the structure feels right in LotR while in XM or ST they are close but not quite there yet.