I think one of the biggest differences between S and A+ is map awareness. You can get to S rank getting away with just coming up with strategies on the spot, but if you don't have a solid game plan and objective for nearly every map and mode (including knowing choke/flank points, points of power/weakness for your weapon and for others, etc), you'll have much less consistency.
Then I think the difference between S and S+ is reading your opponents, implementing strategies much faster, exploiting small weaknesses, and using your advantages better. It is the sum of all the smaller elements, that add up to a large difference. Even with the big difference between S and S+, I still see a ton of tunnel vision in S+, which you want to avoid at all costs. I totally agree though that you need a plan, even if it doesn't work, have a plan, and change it for various factors in each match. As for planning:
For example, yesterday I played a lot on the rotation with Flounder Heights TC. I was playing Custom Hydra, and I had a very clear strategy, and gameplan, that won my squad many matches. It consisted of few, but effective main positions, which included:
1. Play defense from snipe/air conditioner, keep the enemy off tower at that really good choke point right at the start of the tower's path
2. Charge bubbler, then get map control with team, then push tower
3. Keep people from killing team on tower, or ride tower myself
4. Bubble on tower at the choke point if need be
Then as other people mentioned in this thread, you need to improvise, the next parts were fairly improvisational, since they depended on individual actions from every player. I would generally stay on tower until rushed, then jump off, let my teammates jump to me in their street, while defending the jump, and killing enemies approaching tower, also stalling tower. In general I would also plan with my allies to chain invincible specials, but that was my gameplan.
The improvised portions were all strategic in the moment, based upon previous knowledge, and that is the main difference between the ranks: experience. I was able to improvise because of this experience, and also know plans/strategies before the match even began. So the secret is still unfortunately, get gid, in a positive way though, just as always, practice makes perfect, you can't expect to get to S+ with little experience in the game, just like a hobbyist musician can't expect to play with a symphony orchestra. This is why I laugh at ZackScott, who still isn't level 50, and thinks he somehow deserves S+, there is just no way that he has enough experience to be S+, mainly since he hasn't played that much ranked.