I would agree the key to breaking 100 is eliminating penalties and duffs. However, I will say that most over-100 players I play with are terrible putters, particularly at speed control. Even if they get on the green in regulation, they're sometimes taking 3 or 4 putts. While you're certainly going to three putt some at that level, putting should be a focus for improvement.
Key to breaking 90 is just solid play. Solid tee-to-green and solid putting. You don't have to be fantastic at any aspect. Get it on in GIR+1 and 2 putt. You'll hit a few GIR and make a few putts that will get you the pars you need.
While I've only broken 80 once, it seems to me in addition to the solid play tee to green, it's primarily the short game that gets you over the hump. You're still going to miss greens, but a sub-80 player is going to get it up-and-down more often than not.
Key to breaking 90 is just solid play. Solid tee-to-green and solid putting. You don't have to be fantastic at any aspect. Get it on in GIR+1 and 2 putt. You'll hit a few GIR and make a few putts that will get you the pars you need.
While I've only broken 80 once, it seems to me in addition to the solid play tee to green, it's primarily the short game that gets you over the hump. You're still going to miss greens, but a sub-80 player is going to get it up-and-down more often than not.